fix(gateway): coerce scalar free_response_channels to str before split

YAML loads a bare numeric value such as
    discord:
      free_response_channels: 1491973769726791812
as an int.  _discord_free_response_channels() / _slack_free_response_channels()
checked `isinstance(raw, list)` and `isinstance(raw, str)` in that order and
then fell through to `return set()`, so a single-channel config that happened
to be unquoted was silently dropped with no log line — the bot kept demanding
@mentions even though the channel was configured to free-response.

A multi-channel value like `1234567890,9876543210` does not trip this because
the comma forces YAML to parse it as a string.  Single-channel configs are
the only case that breaks, which is exactly the footgun that's hardest to
diagnose (the config "looks right" and the feature just doesn't activate).

Note that the old-schema env-var bridge at gateway/config.py:614+ already
runs `str(frc)` when forwarding to SLACK_/DISCORD_FREE_RESPONSE_CHANNELS,
so the env-var fallback worked.  The bug only surfaces on the
`config.extra["free_response_channels"]` path populated by the `platforms:`
bridge at gateway/config.py:576, which passes the raw YAML value through
unchanged.

Fix at the reader: treat any non-list value as a scalar, coerce with str(),
then apply the same CSV split semantics.  This keeps the public contract
stable (list or str-like continues to work identically) while accepting
the ints that the YAML loader is free to hand us.

Added tests for both Discord and Slack covering:
  - bare int value in config.extra
  - list of ints in config.extra

gateway/platforms/discord.py

@@ -2851,8 +2851,15 @@ class DiscordAdapter(BasePlatformAdapter):
             raw = os.getenv("DISCORD_FREE_RESPONSE_CHANNELS", "")
         if isinstance(raw, list):
             return {str(part).strip() for part in raw if str(part).strip()}
-        if isinstance(raw, str) and raw.strip():
-            return {part.strip() for part in raw.split(",") if part.strip()}
+        # Coerce non-list scalars (str/int/float) to str before splitting.
+        # YAML parses a bare numeric value such as
+        # `free_response_channels: 1491973769726791812` as int, which was
+        # previously falling through the isinstance(str) branch and silently
+        # returning an empty set.  str() here accepts whatever scalar the YAML
+        # loader hands us without changing existing string/CSV semantics.
+        s = str(raw).strip() if raw is not None else ""
+        if s:
+            return {part.strip() for part in s.split(",") if part.strip()}
         return set()
 
     def _thread_parent_channel(self, channel: Any) -> Any:

gateway/platforms/slack.py

@@ -2888,6 +2888,13 @@ class SlackAdapter(BasePlatformAdapter):
             raw = os.getenv("SLACK_FREE_RESPONSE_CHANNELS", "")
         if isinstance(raw, list):
             return {str(part).strip() for part in raw if str(part).strip()}
-        if isinstance(raw, str) and raw.strip():
-            return {part.strip() for part in raw.split(",") if part.strip()}
+        # Coerce non-list scalars (str/int/float) to str before splitting.
+        # A bare numeric YAML value (`free_response_channels: 1234567890`) is
+        # loaded as int and was previously falling through the isinstance(str)
+        # branch to return an empty set.  str() here accepts whatever scalar
+        # the YAML loader hands us without changing existing string/CSV
+        # semantics.
+        s = str(raw).strip() if raw is not None else ""
+        if s:
+            return {part.strip() for part in s.split(",") if part.strip()}
         return set()