test(matrix): make voice-detection tests hermetic against mention gating (#49946)

test_matrix_voice flaked in CI (6/7 failing on some shards, passing on
others and on main) depending on leaked MATRIX_REQUIRE_MENTION env state.

Root cause: the adapter defaults require_mention=True (falling back to the
MATRIX_REQUIRE_MENTION env var). These tests fire a group-room audio event
with no @mention, so _resolve_message_context drops it before dispatch
('No event was captured') whenever require_mention resolves True — which
happens in a clean shard, but an earlier test in another shard can leave
MATRIX_REQUIRE_MENTION=false in os.environ and mask it. The plugin
migration (#5600105478 adapter→bundled plugin) shifted shard composition
and exposed it.

Pin require_mention: False in the test adapter config so these media-TYPE
detection tests are no longer gated by the mention requirement, regardless
of ambient env. Verified: 7/7 pass with MATRIX_REQUIRE_MENTION=true (the
failing condition) AND with the env unset.

tests/gateway/test_matrix_voice.py

@@ -26,7 +26,16 @@ from gateway.platforms.base import MessageType
 # ---------------------------------------------------------------------------
 
 def _make_adapter():
-    """Create a MatrixAdapter with mocked config."""
+    """Create a MatrixAdapter with mocked config.
+
+    Pins ``require_mention: False`` so these media-detection tests are NOT
+    gated by the mention requirement. The adapter defaults require_mention to
+    True (falling back to the MATRIX_REQUIRE_MENTION env var), so without this
+    a group-room audio event with no @mention is dropped by
+    _resolve_message_context before dispatch — making the tests pass or fail
+    depending on leaked env state from other tests in the same shard. These
+    tests exercise voice/audio TYPE detection, not mention gating.
+    """
     from plugins.platforms.matrix.adapter import MatrixAdapter
     from gateway.config import PlatformConfig
 
@@ -36,6 +45,7 @@ def _make_adapter():
         extra={
             "homeserver": "https://matrix.example.org",
             "user_id": "@bot:example.org",
+            "require_mention": False,
         },
     )
     adapter = MatrixAdapter(config)