feat(a2a): implement M12 A2A server

[bug-ops]

Summary

• Add axum-based A2A HTTP server to zeph-a2a crate behind server feature flag
• JSON-RPC 2.0 routing: message/send, tasks/get, tasks/cancel with spec-compliant error codes
• In-memory TaskManager with full task lifecycle (create, get, update, cancel, artifacts, history)
• SSE streaming endpoint (/a2a/stream) with events wrapped in JSON-RPC response envelope
• Bearer token auth with constant-time comparison (subtle::ConstantTimeEq), per-IP rate limiting, 1 MiB body limit
• Agent card served at /.well-known/agent-card.json (public, no auth)
• A2aServerConfig with env var overrides for all fields
• Graceful shutdown via tokio watch channel
• Refactor Part to tagged enum with kind discriminator; rename TaskState::Pending to Submitted with kebab-case serde

Resolves #83, #84, #85
Epic: #82

Test plan

• cargo +nightly fmt --check passes
• cargo clippy --features a2a -- -D warnings passes
• 320 workspace tests pass (21 new server tests + tester-added edge cases)
• Spec compliance verified against a2a-python reference implementation
• Security audit: constant-time auth, body limits, rate limiting, no unsafe
• Code review: approved with no blocking issues

[codecov-commenter]

Codecov Report

❌ Patch coverage is 89.00929% with 142 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
src/main.rs	0.00%	54 Missing ⚠️
crates/zeph-a2a/src/server/mod.rs	89.28%	51 Missing ⚠️
crates/zeph-a2a/src/server/handlers.rs	94.39%	12 Missing ⚠️
crates/zeph-core/src/config.rs	72.72%	12 Missing ⚠️
crates/zeph-a2a/src/types.rs	88.33%	7 Missing ⚠️
crates/zeph-a2a/src/server/state.rs	98.31%	4 Missing ⚠️
crates/zeph-a2a/src/server/router.rs	99.02%	2 Missing ⚠️

@@            Coverage Diff             @@
##             main      #98      +/-   ##
==========================================
+ Coverage   78.93%   81.05%   +2.11%     
==========================================
  Files          28       32       +4     
  Lines        4881     6161    +1280     
==========================================
+ Hits         3853     4994    +1141     
- Misses       1028     1167     +139     

Files with missing lines	Coverage Δ
crates/zeph-a2a/src/client.rs	84.25% <100.00%> (+0.13%)	⬆️
crates/zeph-a2a/src/error.rs	97.05% <ø> (ø)
crates/zeph-a2a/src/server/router.rs	99.02% <99.02%> (ø)
crates/zeph-a2a/src/server/state.rs	98.31% <98.31%> (ø)
crates/zeph-a2a/src/types.rs	97.08% <88.33%> (-2.92%)	⬇️
crates/zeph-a2a/src/server/handlers.rs	94.39% <94.39%> (ø)
crates/zeph-core/src/config.rs	93.73% <72.72%> (-2.73%)	⬇️
crates/zeph-a2a/src/server/mod.rs	89.28% <89.28%> (ø)
src/main.rs	0.00% <0.00%> (ø)

... and 1 file with indirect coverage changes

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