test: blank platform-gating env vars in hermetic fixture

load_gateway_config() has a side effect: when config.yaml contains
platform-gating keys (slack.require_mention, slack.strict_mention,
slack.free_response_channels, slack.allow_bots, slack.reactions, plus
analogous keys for discord/telegram/whatsapp/dingtalk/matrix), it calls
os.environ[KEY] = ... to bridge them to env-var form.

monkeypatch.delenv doesn't track direct os.environ mutations made
inside the test body, so tests that call load_gateway_config() leak
those env vars into later tests on the same xdist worker. The failure
mode is flaky seed-dependent: test_top_level_message_requires_mention_
even_with_session (and siblings in TestThreadReplyHandling) pass when
SLACK_REQUIRE_MENTION is unset but fail when a leaked value of 'false'
is present.

Add the gating env vars to _HERMES_BEHAVIORAL_VARS so the hermetic
autouse fixture blanks them on every test setup, closing the leak
regardless of which test sets them.

tests/conftest.py

@@ -211,6 +211,21 @@ _HERMES_BEHAVIORAL_VARS = frozenset({
     "SIGNAL_ALLOW_ALL_USERS",
     "EMAIL_ALLOW_ALL_USERS",
     "SMS_ALLOW_ALL_USERS",
+    # Platform gating — set by load_gateway_config() as a side effect when
+    # a config.yaml is present, so individual test bodies that call the
+    # loader leak these values into later tests on the same xdist worker.
+    # Force-clear on every test setup so the leak can't happen.
+    "SLACK_REQUIRE_MENTION",
+    "SLACK_STRICT_MENTION",
+    "SLACK_FREE_RESPONSE_CHANNELS",
+    "SLACK_ALLOW_BOTS",
+    "SLACK_REACTIONS",
+    "DISCORD_REQUIRE_MENTION",
+    "DISCORD_FREE_RESPONSE_CHANNELS",
+    "TELEGRAM_REQUIRE_MENTION",
+    "WHATSAPP_REQUIRE_MENTION",
+    "DINGTALK_REQUIRE_MENTION",
+    "MATRIX_REQUIRE_MENTION",
 })