fix: bound reasoning cache writes

[guillaumegay13]

Summary

• cap reasoning cache values at 256 KiB and local content at 64 MiB
• limit shared-cache persistence to four concurrent and 256 queued writes
• coalesce queued updates by session and tool call

Testing

• reasoning content cache unit tests
• backend build
• ESLint and changeset validation

---

Summary by cubic

Bound reasoning cache memory and DB write throughput to prevent stalls and memory bloat during database backpressure. Adds size caps, write coalescing, and concurrency limits for shared-cache persistence.

• Bug Fixes
  • Cap single reasoning_content to 256 KiB; drop oversized values (local and shared).
  • Cap local cache to 64 MiB and 10k entries; evict oldest by FIFO with byte tracking.
  • Limit shared-cache persistence to 4 concurrent writes and 256 queued; coalesce by session + tool call and keep only the latest.
  • Clear pending persists on session purge and skip expired entries; keep warming local cache on reads.

^{Written for commit ea4e865. Summary will update on new commits.}

[codecov]

Codecov Report

❌ Patch coverage is 92.59259% with 4 lines in your changes missing coverage. Please review.
✅ Project coverage is 96.39%. Comparing base (df19e6e) to head (ea4e865).

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2636      +/-   ##
==========================================
- Coverage   96.40%   96.39%   -0.01%     
==========================================
  Files         600      600              
  Lines       35238    35282      +44     
  Branches     9751     9811      +60     
==========================================
+ Hits        33972    34011      +39     
- Misses        704      709       +5     
  Partials      562      562              

Flag	Coverage Δ
backend	95.62% <92.59%> (-0.02%)	⬇️
frontend	97.21% <ø> (ø)
shared	99.43% <ø> (ø)

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