fix(conversations): compare-and-swap the denormalized message cache

[leseb]

Summary

Concurrent conversation item appends each rebuilt the denormalized messages cache from the authoritative item rows and wrote it back unconditionally, so a slower writer could overwrite a newer writer's cache with a stale snapshot and drop items from the history that rehydration consumes. This replaces the blind write with a compare-and-swap that retries a lost race with a freshly read snapshot, and reuses the create/delete caller's existing pre-mutation snapshot to skip a redundant cache reload; because the written value is always a fresh rebuild from item rows, a stale snapshot only ever loses the swap and never persists stale data. Exhausting the bounded retries logs and returns Ok since the row mutation is already durably committed and any later write rebuilds the cache, while genuine store errors and a disappeared conversation still propagate.

Related issue

Closes #662

Validation

• Unit tests — cargo test -p praxis-ai-apis (2592 passed, 0 failed; six Concurrent item appends can overwrite the denormalized conversation history #662 cache-sync tests covering stale-cache repair, concurrent-clobber, snapshot fast-path with no reload, stale-snapshot fresh-read fallback, missing-conversation error, and retry exhaustion)
• Integration or functional tests — n/a; the concurrency is exercised deterministically via a fault-injecting store wrapper over the real SQLite store, and there is no new config surface to add a functional example test for
• make lint and make build

Checklist

• I reviewed every changed line and can explain the change.
• New capabilities include an example config and functional example test. — n/a; bug fix, no new capability or config surface
• User-facing behavior and generated documentation are updated. — n/a; internal cache-consistency behavior only
• Performance-sensitive changes include appropriate benchmark or load-test evidence. — n/a; correctness fix (bounded CAS retries only on contention)
• Commits are signed and include a Signed-off-by trailer.

Breaking changes

None. sync_conversation_messages is a crate-internal (pub(super)) helper; the signature change is not part of any public API.

[franciscojavierarceo]

reviewed the current head, all available comments and review threads, the architectural paths, and the completed checks. i found no actionable issues. approving this commit.