[TAN-7901] Test suite for the mcp engine

[adessy]

The MCP engine was built velocity-first with minimal test coverage. This PR adds a systematic test suite, plus the production fixes the suite surfaced.

Production fixes (found while writing the suite)

• Missing authorization guards — update_project, update_phase, update_resource and replace_form_fields called neither authorize_project! nor Pundit authorize, so the draft-only constraint was not enforced for updates: a super admin could mutate published projects through the MCP.
• Broken get_form_fields → replace_form_fields round-trip — Serializers::CustomField emitted a constraints key that is not a CustomField attribute; echoing fetched fields back crashed UpdateAllService. The serializer now drops the key (constraints are reported at the top level).
• Silent empty list for a bad parent id — list_events and list_causes filtered without looking up the parent, so a nonexistent project_id/phase_id returned [] (an LLM reads that as "no events" rather than "wrong id"). They now return the standard not-found error like their siblings.
• No way to remove a project header background — remote_header_bg_url was a non-nullable string; it now accepts null, which update_project translates into remove_header_bg! (CarrierWave ignores plain nil assignment), mirroring the web API's remove_image_if_requested!.
• Group visibility was invisible and unreachable — with visible_to: 'groups', the project payload carried no group information, the schemas had no way to set the group list, and list_groups' project_id filter matched smart-group rules referencing the project rather than the groups that can see it. The project schemas now accept group_ids (replace-wholesale, like area_ids), the serializer surfaces group_ids, and the misleading filter is removed (read ids from the payload, resolve names via list_groups).
• Opaque replace_form_fields validation errors — form-level error keys (stale_data, locked_deletion, …) were relayed as a raw JSON dump; they now map to actionable prose (re-fetch and retry, echo locked fields back, structural page rules), with the raw errors still in structuredContent.
• Settable but not clearable fields — end_at (the only way to make the last phase open-ended), access_denied_explanation_multiloc and uploader files (remote_<x>_url) now accept null to clear. Uploader clearing is a generic Runner helper driven by CarrierWave's mount registry (nil assignment is otherwise silently ignored); the permission multiloc also now merges per locale like every other multiloc field instead of replacing wholesale.
• Unreachable CarrierWave rescues removed — download failures never raise out of save! (CarrierWave converts them into :carrierwave_download_error validation errors on assignment), so the dedicated rescues in the attach tools were dead code on every path.
• Mass-assignment through undeclared schema keys — JSON Schema accepts undeclared keys by default and the MCP gem passes arguments to tools verbatim (verified at runtime), so create_phase (Phase.new(**params)) and replace_form_fields (assign_attributes via UpdateAllService — including resource_id, which re-parents a field to another form) were mass-assignable. Their schemas are now closed with additionalProperties: false. To keep the fetch → edit → replace round-trip working, the form-field schema is now shared between both tools with standard readOnly/writeOnly annotations on the directional keys; BaseTool.for strips readOnly keys from the params at dispatch (generic schema walker, unit-tested as a function), and a registry tripwire fails with the offending JSON-Pointer path if a readOnly ever lands where the walker can't reach it (only properties/items positions are supported — full coverage would take a JSON Schema evaluator).
• Small cleanups — the BaseTool::Authorization concern is gone (its pundit_user was shadowed by Pundit::Authorization's identical default; the Runner now includes Pundit directly), and the draft-guard refusal no longer ends in a doubled period (reasons own their punctuation; the template only terminates its own fallback).

Refactors that rode along

• Single serialization pathway — paginated_response's as_json(only: …) fast path is gone; the three flat-payload tools got from-scratch serializers (no wraps), and Serializers::Base#to_h enforces symbols-as-top-level-keys so payload key types can't diverge.
• Argument validation made explicit — the controller passes MCP::Configuration.new(validate_tool_call_arguments: true) instead of relying on the gem default; Runners trust schema-validated input, and a request-spec example pins the rejection end to end.

Coverage (lines, engines/commercial/mcp_server/app/**)

	Baseline	This PR
Total	740/887 = 83.4%	954/956 = 99.8%
Examples	76	285

The two uncovered lines are defensive branches (an unreachable RecordInvalid rescue in replace_form_fields, the non-redirectable branch of the OAuth authorize endpoint).

Notes / follow-ups (not addressed here)

• Make additionalProperties: false structural instead of per-tool discipline.
• List tools should go through Pundit policy scopes (policy_scope) instead of querying unscoped. Today McpPolicy restricts the endpoint to super admins so nothing leaks, but scoping would make the invariant local.

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Changelog

Technical

• [TAN-7901] Test suite for the MCP server engine (tools, serializers, base modules, HTTP dispatch): 76 → 285 examples.

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🧪 [MCP] Implement tests

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[adessy]

@amanda-anderson @kogre for whoever has the time... or the courage.

[amanda-anderson]

LGTM! Didn't look as deeply into the individual specs themselves, but the ones I did look at made sense to me :)