fix: invalidate stale thread participants when recipients change

[naveensingh]

Type of change(s)

• Bug fix
• Feature / enhancement
• Infrastructure / tooling (CI, build, deps, tests)
• Documentation

What changed and why

• Fix stale thread participant cache when a thread's recipient identity changes.
• Refresh conversation participants from when opening or refreshing a thread.

Tests performed

• I could never reproduce the original bug, but I have tested sending SMS messages to various contacts and normal app operation.

Closes the following issue(s)

• Closes App insists on answering to wrong contact #615
• Maybe Reply to group message is sent to unrelated recipient #595
• Maybe Conversation does not match contact after removing focus from app #365

Checklist

• I read the contribution guidelines.
• I manually tested my changes on device/emulator (if applicable).
• I updated the "Unreleased" section in CHANGELOG.md (if applicable).
• I have self-reviewed my pull request (no typos, formatting errors, etc.).
• I understand every change in this pull request.