Add updateMessage handler on ThreadStorage

[AbhinRustagi]

What

Fixes TH-2025.

1. Updated the docs to mention the new updateMessage method on the ThreadStorage interface.
2. Added a replaceMessageId on processStreamedMessage which earlier generated a client-side UUID for each message for the render session. However, it does not replace it with the server-side generated ID until the data is restored from the API. Because of that, during render, a updateMessage would result in a 404 since the message IDs don't match. The function replaces the ID during streaming as soon as the TEXT_MESSAGE_START event is received which carries the server generated message ID.
3. Added a ref check in GenUIAssistantMessage.tsx which would check if the last persisted message matches the current and avoids refiring the updateMessage flow since the onStateUpdate is called by the function itself and by store subscription as well).

Checklist

• I linked a related issue, if applicable
• I updated docs/README when needed
• I considered backwards compatibility