## Why
MCP configuration can change while a thread is still starting, before it appears in the thread manager's loaded-thread list. In that window, invalidation could miss the new thread and leave it with a stale MCP runtime.
## What changed
- Centralize MCP runtime invalidation in `ThreadManager` so it refreshes loaded threads and marks in-progress thread startups for refresh after publication.
- Use the centralized invalidation path for account and plugin changes.
- Invalidate MCP runtimes after successful server and plugin OAuth logins.
## Testing
- Add a regression test that pauses thread startup, invalidates MCP runtimes, and verifies that the newly published thread refreshes its MCP projection.
GitOrigin-RevId: 8944f5a21816a1405e038a0d4c08b66362ea9368
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