fix(workspace): restore cache-short-circuit in enrich_peer_metadata_nonblocking

PR #502 removed the cache check from enrich_peer_metadata_nonblocking
to "make test isolation deterministic", but this broke 5 tests that
depended on the cache-hit path:

- test_envelope_enrichment_uses_cache_when_present:
  KeyError: 'peer_name' (cache populated but not read)
- test_envelope_enrichment_fetches_on_cache_miss:
  KeyError: 'peer_name' (second push expected warm cache)
- test_envelope_enrichment_re_fetches_after_ttl:
  KeyError: 'peer_name' (stale TTL expected to re-fetch)
- test_enrich_peer_metadata_nonblocking_cache_hit_returns_immediately:
  assert None is not None (expected record on cache hit)
- test_enrich_peer_metadata_nonblocking_cache_miss_schedules_fetch:
  assert None is not None (expected record on second call)

Restore the cache check inside the _enrich_in_flight_lock critical
section. The lock is necessary because without it, a cache-hit thread
and a cache-miss thread could both pass the in-flight gate before
either populates the cache, spawning two parallel GETs for the same
peer. The cache check inside the lock serialises this correctly.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

workspace/a2a_client.py

@@ -187,12 +187,21 @@ def enrich_peer_metadata_nonblocking(
     canon = _validate_peer_id(peer_id)
     if canon is None:
         return None
+    current = time.monotonic()
     # Schedule background fetch unless one is already in flight for this
     # peer. The synchronous version atomically reads-then-writes; the
     # async version splits that into "schedule fetch" + "fetch fills
     # cache later." The in-flight set keeps a flurry of pushes from
     # one peer (e.g., a chatty agent) from spawning N parallel GETs.
+    # The cache check lives inside the lock so a concurrent cache-hit
+    # thread and a cache-miss thread don't both pass the in-flight gate
+    # before either has populated the cache.
     with _enrich_in_flight_lock:
+        cached = _peer_metadata_get(canon)
+        if cached is not None:
+            fetched_at, record = cached
+            if current - fetched_at < _PEER_METADATA_TTL_SECONDS:
+                return record
         if canon in _enrich_in_flight:
             return None
         _enrich_in_flight.add(canon)