fix(relay): coerce Slack behavior flags exactly as the native adapter does

Both relay Slack knobs read their value through bool(), while the native
adapter they mirror uses str(raw).strip().lower() in {"1","true","yes","on"}.
A YAML-quoted string diverges:

    dm_top_level_threads_as_sessions: "false"   → relay True, native False

Non-empty strings are truthy, so the escape hatch is silently ignored in
exactly the shape an operator writes to switch it OFF. reply_in_thread has the
same defect and gates reply placement, session keying and run.py's progress
resolver, so one quoted "false" misfires three ways.

Route both through a shared _coerce_flag mirroring native's predicate. Real
booleans pass through untouched; None falls back to the default. Contract §8
documents the accepted spellings.

Tests: both knobs parametrized over the true/false spellings native accepts,
plus the absent-key default.

docs/relay-connector-contract.md

@@ -731,6 +731,10 @@ platforms:
 Resolution: nested `extra.<platform>` object wins → legacy flat key on
 `extra` honored as fallback → default. Source of truth:
 `RelayAdapter._effective_reply_in_thread` (`gateway/relay/adapter.py`).
+Values coerce exactly as the native Slack adapter's do — `1/true/yes/on`
+(case-insensitive, whitespace-trimmed) are ON, anything else is OFF — so a
+YAML-quoted `"false"` turns a knob off rather than being read as a truthy
+string.
 
 Current controls (Slack):
 
@@ -745,6 +749,13 @@ thread-scoped, and in flat mode the send-side anchor strip guarantees the
 status anchor can never leak into reply placement. Semantics of the native
 key: see `website/docs/user-guide/messaging/slack.md`.
 
+Thread-anchor resolution applies to EVERY send lane — text (`send`) and media
+(`send_media`) alike — through one choke point
+(`RelayAdapter._apply_slack_thread_anchor`). Media frames egress via the same
+connector-side Slack sender, which threads on `metadata.thread_id` only, so an
+attachment resolves its anchor identically to a text reply: promoted into
+metadata in thread-per-message mode, stripped in flat mode.
+
 Changes take effect on gateway restart; no connector involvement.
 
 ---

gateway/relay/adapter.py

@@ -344,10 +344,30 @@ class RelayAdapter(BasePlatformAdapter):
         sub = extra.get("slack")
         return sub if isinstance(sub, dict) else extra
 
+    @staticmethod
+    def _coerce_flag(raw: Any, default: bool) -> bool:
+        """Coerce an operator-supplied boolean exactly as native Slack does.
+
+        Native SlackAdapter reads its behavior flags with
+        ``str(raw).strip().lower() in {"1","true","yes","on"}``, so a
+        YAML-quoted ``"false"`` — a shape operators write routinely — turns
+        the flag OFF. A bare ``bool()`` would read that same string as True
+        (non-empty string), silently ignoring the off switch. These knobs are
+        documented as native-parity mirrors, so they must coerce identically
+        or the parity claim only holds for unquoted YAML booleans.
+        """
+        if raw is None:
+            return default
+        if isinstance(raw, bool):
+            return raw
+        return str(raw).strip().lower() in {"1", "true", "yes", "on"}
+
     def _effective_reply_in_thread(self) -> bool:
         """Resolve the thread-per-message vs flat-DM mode for fronted Slack."""
         try:
-            return bool(self._relay_slack_extra().get("reply_in_thread", True))
+            return self._coerce_flag(
+                self._relay_slack_extra().get("reply_in_thread"), True
+            )
         except Exception:  # noqa: BLE001 - config shape is operator-owned
             return True
 
@@ -363,10 +383,9 @@ class RelayAdapter(BasePlatformAdapter):
         legacy steer/queue posture, decoupled from reply_in_thread.
         """
         try:
-            return bool(
-                self._relay_slack_extra().get(
-                    "dm_top_level_threads_as_sessions", True
-                )
+            return self._coerce_flag(
+                self._relay_slack_extra().get("dm_top_level_threads_as_sessions"),
+                True,
             )
         except Exception:  # noqa: BLE001 - config shape is operator-owned
             return True

tests/gateway/relay/test_relay_slack_dm_streaming.py

@@ -341,6 +341,48 @@ async def test_media_caller_metadata_not_mutated():
     assert caller_md == {"user_id": "U1"}, "caller metadata was mutated in place"
 
 
+# ---------------------------------------------------------------------------
+# Operator flags coerce exactly as the native Slack adapter's do.
+# ---------------------------------------------------------------------------
+@pytest.mark.parametrize(
+    "raw,expected",
+    [
+        (False, False),
+        ("false", False),
+        ("False", False),
+        (" no ", False),
+        ("off", False),
+        ("0", False),
+        (True, True),
+        ("true", True),
+        ("yes", True),
+        ("on", True),
+        ("1", True),
+    ],
+)
+def test_relay_slack_flags_coerce_like_native(raw, expected):
+    """A YAML-quoted "false" must turn these knobs OFF, matching native's
+    str().strip().lower() predicate. A bare bool() would read any non-empty
+    string as True and silently ignore the operator's off switch."""
+    adapter, _stub = _wire("D1", "dm")
+    adapter.config.extra = {
+        "slack": {
+            "reply_in_thread": raw,
+            "dm_top_level_threads_as_sessions": raw,
+        }
+    }
+    assert adapter._effective_reply_in_thread() is expected
+    assert adapter._dm_top_level_threads_as_sessions() is expected
+
+
+def test_relay_slack_flags_default_true_when_absent():
+    """Both knobs default ON when the operator sets nothing."""
+    adapter, _stub = _wire("D1", "dm")
+    adapter.config.extra = {}
+    assert adapter._effective_reply_in_thread() is True
+    assert adapter._dm_top_level_threads_as_sessions() is True
+
+
 # ---------------------------------------------------------------------------
 # The status clear targets the same thread the heartbeat set.
 # ---------------------------------------------------------------------------