fix(tests): handle CID mismatch error format and shorten community name
- Fix concurrent modification test to accept either ErrConflict (409)
or CID mismatch error (400 "Record was at")
- Shorten e2epost community name to stay under handle length limit
Remaining: TestFullUserJourney_E2E fails due to PDS token validation
issue ("InvalidToken") - this is a test infrastructure issue where the
community's stored access token is no longer valid with the PDS.
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Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>