fix(a2a): security hardening from code review
Critical fixes:
- SSRF protection: validate push notification callback URLs (block
internal/private/loopback/metadata, enforce http/https only)
- Request body size limit: 1MB max (prevents memory exhaustion DoS)
- Thread safety: module-level locks for turn tracking, rate limiting,
and pending task registry (was lazily initialized, racy)
- Peer identity: fall back to client IP when 'peer' field absent
(prevents rate limiting collapse to single 'unknown' bucket)
Minor fixes:
- Watchdog survives reconnect: clear _watchdog_stop in connect()
- Redact error messages before sending to peers
- Remove dead _streaming_queues state
- Fix duplicate tags key in Agent Card skills
- Always send contextId in a2a_call (fixes client/server mismatch)
- Clear push_callbacks on disconnect
- SSE streaming cleanup via try/finally
16 new tests covering SSRF, body size, thread safety, watchdog
reconnect, error redaction, contextId consistency.
Tests: 97 passed, 3 deselected, 0 failed.
37481dccf4