fix: message parts - use partIds and helpers instead of unstable indices (1/7)

[DeveloperBlue]

Message-parts APIs currently incorrectly treat parts list indices as if they were iMessage part IDs. After edits, unsends, or gallery collapse, anything that keys off those values (replies, threads, swipe-to-reply, bubble chrome, and reactions) can target the wrong part or break.

This PR adds helpers to look up parts by id (partById, coversPartId), attachmentGuid for replies to a specific gallery attachment, and leading/trailing helpers (isLeadingMessagePart / isTrailingMessagePart) so collapsed galleries still get the correct tails, connect styling, and reaction/sticker placement.

Bugs Addressed:

• Tapbacks/stickers on a photo in a collapsed gallery applied to / rendered on the wrong attachment (or didn’t appear)
• Popover Reply on a gallery image replied to the wrong part and previewed the wrong attachment
• Reply-thread open / reply-bubble render crashed or showed the whole parent message when the target part was missing
• Bubble effects, tails, avatars, and Material multi-bubble connectors broke on collapsed galleries because leading/trailing used list index instead of the gallery’s covered part ids
• “Reply recent” could target the wrong part when part ids don’t match parts.length - 1
• Reply-thread line decoration (and originator thread open via a gallery part) missed replies whose thread part id sat inside a collapsed gallery span; bubble-level “X Replies” on galleries is suppressed in favor of per-attachment reply UX

Each architecture change is commit-by-commit, so you can review each commit individually for the scoped changes:
partById implementation → partById call sites → attachmentGuid for reply contexts→ null guards for missing messageParts→ leading/trailing part helpers → per-part/per-attachment reaction and sticker matching → threadsForParts so reply threads match any part id in a collapsed gallery span.

see file changes

1. add partById to resolve message parts by id instead of unstable list index

Adds MessageState.partById and switches reply/thread UI from parts[index] to lookup by MessagePart.part, so targeting survives edits, unsends, and gallery collapse.

• lib/app/state/message_state.dart — Add partById (lookup by MessagePart.part)
• lib/app/layouts/conversation_view/widgets/message/message_holder.dart — Resolve reply-target parts by id
• lib/app/layouts/conversation_view/widgets/message/reply/reply_bubble.dart — Load the replied-to part by id
• lib/app/layouts/conversation_view/widgets/text_field/reply_holder.dart — Reply composer preview uses id lookup
• lib/app/layouts/conversation_view/widgets/message/popup/actions/navigation_actions.dart — Thread originator resolved by id

2. fix: use message-part ids for swipe-to-reply, bubble effects, reply-recent, and gallery cards

Propagates part ids through the remaining interaction surfaces, and records original part ids per attachment when consecutive media parts collapse into a gallery.

• lib/database/global/message_part.dart — partIndexForAttachment / attachmentPartIndices for collapsed galleries
• lib/app/layouts/conversation_view/widgets/message/attachment/message_image_gallery.dart — Per-card swipe/reply uses each attachment’s original part id
• lib/app/layouts/conversation_view/widgets/message/message_holder.dart — Swipe-to-reply / bubble effects use part ids
• lib/app/layouts/conversation_view/widgets/message/misc/message_part_content.dart — Pass part id through content wrapper
• lib/services/backend_ui_interop/intents.dart — Reply Recent targets parts.last.part, not parts.length - 1

3. fix: plumb attachmentGuid through reply context to resolve specific attachments

Extends reply context so a reply can target a specific attachment inside a multi-attachment part/gallery, not just the part id.

• lib/models/message_reply_context.dart — Optional attachmentGuid on reply context
• lib/app/layouts/conversation_view/widgets/message/misc/swipe_to_reply_wrapper.dart — Carry attachmentGuid into MessageReplyContext
• lib/app/layouts/conversation_view/widgets/message/popup/actions/navigation_actions.dart — Popover reply sets guid when the part has a single attachment
• lib/app/layouts/conversation_view/widgets/text_field/reply_holder.dart — Preview resolves by guid first, then falls back to partById

4. fix: guard reply/thread paths when a reply targets a missing message part

Makes reply/thread paths null-safe when partById misses (deleted, unsent, or out-of-sync parts) instead of force-unwrapping.

• lib/app/layouts/conversation_view/widgets/message/message_holder.dart — Null-safe matched part when rendering a reply target
• lib/app/layouts/conversation_view/widgets/message/reply/reply_bubble.dart — Nullable part; skip/fallback instead of !
• lib/app/layouts/conversation_view/widgets/message/popup/actions/navigation_actions.dart — Guard thread open when originator part is missing
• lib/app/layouts/conversation_view/widgets/message/misc/swipe_to_reply_wrapper.dart — Align with safer reply-context construction
• lib/models/message_reply_context.dart — Clarify partIndex as message-part id

5. fix: add isLeading/isTrailingMessagePart helpers for multipart layouts

Replaces list-position leading/trailing checks with span-aware helpers so bubble tails, connect styling, and chrome stay correct for collapsed galleries.

• lib/database/global/message_part.dart — coversPartId for single parts and gallery spans
• lib/app/state/message_state.dart — isLeadingMessagePart / isTrailingMessagePart via coversPartId
• lib/app/layouts/conversation_view/widgets/message/message_holder.dart — Stickers, effects, connectUpper, tails use leading/trailing helpers
• lib/app/layouts/conversation_view/widgets/message/parts/message_part_wrapper.dart — Same for bubble wrapper chrome
• lib/app/layouts/conversation_view/widgets/message/attachment/attachment_holder.dart — Tail only on trailing part
• lib/app/layouts/conversation_view/widgets/message/message_holder/message_holder_timestamps.dart — Edit-history bubble chrome matches leading/trailing

6. fix: target correct message parts for bubble reactions/stickers for collapsed galleries via coversPartId

Switches reaction/sticker matching to coversPartId so associations against any id in a collapsed gallery span still render on that bubble.

• lib/app/layouts/conversation_view/widgets/message/message_holder.dart — reactionsForPart takes MessagePart and filters via coversPartId
• lib/app/layouts/conversation_view/widgets/message/message_holder/message_holder_reactions.dart — Reactions, stickers, and spacing use span-aware matching
• lib/app/layouts/conversation_view/widgets/message/message_holder/message_holder_timestamps.dart — Samsung timestamp padding uses the updated callback
• lib/app/layouts/conversation_view/widgets/message/message_holder/message_reactions.dart — Reaction widget callback signature matches

7. fix: resolve reply threads by message-part id across collapsed gallery part spans

Adds a multi-part thread lookup so replies keyed to any id covered by a collapsed gallery are found together, and keeps gallery reply UX per-attachment (native iMessage).

• lib/database/global/chat_messages.dart — threadsForParts matches normalizedThreadPart against any originator part id in the set
• lib/app/layouts/conversation_view/widgets/message/message_holder.dart — Reply line decoration uses threadsForParts with attachmentPartIndices
• lib/app/layouts/conversation_view/widgets/message/misc/message_properties.dart — Skip aggregate reply count on media galleries; otherwise use threadsForParts
• lib/app/layouts/conversation_view/widgets/message/reply/reply_thread_popup.dart — Opening a thread from a gallery originator unions replies across covered part ids

Groundwork for addressing #3122, #2605, and #2812

[DeveloperBlue]

First chunk of #3147 for #3122

Each commit is specifically scoped, so hopefully reviewing should be smooth.

[DeveloperBlue]

Ready to go, let me know if the PR changes are a good size