Clarify session eviction cleanup guarantees

[JAORMX]

Summary

• To avoid promising teardown completion that the shared eviction drain cannot guarantee, clarify that callbacks run without cache mutexes held by their invoking goroutine and may be drained by another operation.
• To keep session lifecycle documentation consistent with Close terminated vMCP sessions instead of waiting for eviction #6807, distinguish eager local cache removal from completion of backend cleanup and update the older termination test's lazy-eviction comments.

Addresses the three non-blocking review comments on #6807. This follow-up changes comments only; synchronization, teardown behavior, and test assertions are unchanged.

Type of change

• Bug fix
• New feature
• Refactoring (no behavior change)
• Dependency update
• Documentation
• Other (describe):

Test plan

• Unit tests (task test)
• E2E tests (task test-e2e)
• Linting (task lint-fix)
• Manual testing (describe below)

Reviewed the documented callback ownership and return-time guarantees against bufferEvicted, drainEvictions, cache mutation paths, and Manager.Terminate. Ran git diff --check and a diff audit confirming that every changed line is a Go comment or blank line. Local unit/E2E tests were not run because executable code and assertions are unchanged.

Does this introduce a user-facing change?

No. Documentation only.

Implementation plan

Approved implementation plan

Merge the focused session-retention fix in #6807 without holding it for documentation polish, then address its three non-blocking comments in a separate PR:

1. Qualify termination's backend-teardown completion guarantee.
2. Clarify callback lock ownership and shared-drain latency semantics.
3. Update the older test comments that still describe lazy eviction.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 79.63%. Comparing base (607cbe8) to head (81ee385).

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #6833      +/-   ##
==========================================
+ Coverage   79.62%   79.63%   +0.01%     
==========================================
  Files         815      815              
  Lines       82965    82965              
==========================================
+ Hits        66060    66073      +13     
+ Misses      16900    16887      -13     
  Partials        5        5              

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