implement TTL-based eviction for A2A rate limiter

[bug-ops]

Summary

• Add periodic background task (every 60s) that evicts stale entries from rate limiter HashMap
• Add max entries cap (10,000) with full clear as defense-in-depth
• Add unit tests for eviction and cap behavior

Test plan

• cargo nextest run -p zeph-a2a — 71 tests pass
• cargo clippy --workspace -- -D warnings — clean

Closes #306

[codecov-commenter]

Codecov Report

❌ Patch coverage is 88.13559% with 7 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
crates/zeph-a2a/src/server/router.rs	88.13%	7 Missing ⚠️

@@            Coverage Diff             @@
##             main     #321      +/-   ##
==========================================
+ Coverage   79.96%   80.02%   +0.06%     
==========================================
  Files          98       98              
  Lines       23964    23982      +18     
==========================================
+ Hits        19163    19192      +29     
+ Misses       4801     4790      -11     

Files with missing lines	Coverage Δ
crates/zeph-a2a/src/server/router.rs	96.65% <88.13%> (-2.41%)	⬇️

... and 3 files with indirect coverage changes

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