Add LRU capacity to ValidatingCache, remove sentinel pattern, add storage Update

[yrobla]

Summary

Re-applies the changes from #4669 which were reverted in #4730.

• Reintroduces pkg/cache.ValidatingCache[K,V]: a fully-typed, capacity-bounded LRU cache using mutex+map+container/list
• Removes the terminatedSentinel / sentinel+rollback pattern
• Adds DataStorage.Update (SET XX semantics) to close the TOCTOU resurrection window
• Adds FactoryConfig.CacheCapacity to wire the LRU limit through sessionmanager.New
• Cache tests live in pkg/cache/validating_cache_test.go

Fixes #4494

Type of change

• New feature

Test plan

• Unit tests (task test)
• Linting (task lint-fix)

🤖 Generated with Claude Code

Large PR Justification

This is the revert of an existing PR. Cannot be split.

[github-actions]

Large PR Detected

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## Large PR Justification

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[jerm-dro]

Some failing tests that seem minor.

I'm assuming there's no major changes here compared to the original, but LMK if that's not true

[codecov]

Codecov Report

❌ Patch coverage is 78.57143% with 33 lines in your changes missing coverage. Please review.
✅ Project coverage is 68.86%. Comparing base (acc1f66) to head (aecdae1).
⚠️ Report is 25 commits behind head on main.

Files with missing lines	Patch %	Lines
pkg/vmcp/server/sessionmanager/session_manager.go	57.44%	10 Missing and 10 partials ⚠️
pkg/cache/validating_cache.go	80.00%	5 Missing and 5 partials ⚠️
...kg/transport/session/session_data_storage_redis.go	82.35%	2 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #4731      +/-   ##
==========================================
+ Coverage   68.75%   68.86%   +0.11%     
==========================================
  Files         518      517       -1     
  Lines       54826    54470     -356     
==========================================
- Hits        37693    37512     -181     
+ Misses      14239    14085     -154     
+ Partials     2894     2873      -21     

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Large PR justification has been provided. Thank you!

[github-actions]

✅ Large PR justification has been provided. The size review has been dismissed and this PR can now proceed with normal review.