fix: slim the Slack edit-failure log to one line via the existing payload helper

Follow-up to the salvaged #111938 commit: `_slack_response_payload` already normalizes a
SlackResponse/dict body, so the new `_slack_api_error_code` helper and the two-branch
logger.error were redundant. One log line now always carries `api_error=<code|none>` so an
HTTP 200 + ok=false failure (e.g. message_not_found) is readable without exc_info.

Tests trimmed to one invariant per fix (session key on the response-ready line; API error
code on the edit failure); the `session=unknown` fallback test was a change-detector.

plugins/platforms/slack/adapter.py

@@ -955,16 +955,6 @@ def _is_transient_transport_error(e: BaseException) -> bool:
         and not is_permanent_tls_error)
 
 
-def _slack_api_error_code(e: BaseException) -> Optional[str]:
-    """Return Slack's API error code when an SDK response exposes one."""
-    response = getattr(e, "response", None)
-    data = getattr(response, "data", None)
-    if not isinstance(data, dict):
-        return None
-    error = data.get("error")
-    return str(error) if error else None
-
-
 def _extra_or_env_flag_getter(key: str, env_var: str, *, strip: bool = False) -> Callable[..., bool]:
     """Method factory: ``self._extra_or_env_flag(key, env_var, strip=strip)``."""
 
@@ -2357,15 +2347,12 @@ class SlackAdapter(BasePlatformAdapter):
                     message_id, chat_id, e, exc_info=True)
                 return SendResult(
                     success=False, error=str(e), retryable=True, error_kind="transient")
-            api_error = _slack_api_error_code(e)
-            if api_error:
-                logger.error(
-                    "[Slack] API chat.update failure on message %s in channel %s: api_error=%s: %s",
-                    message_id, chat_id, api_error, e, exc_info=True)
-            else:
-                logger.error(
-                    "[Slack] Failed to edit message %s in channel %s: %s", message_id, chat_id, e,
-                    exc_info=True)
+            # An HTTP 200 + ``ok=false`` reply raises too; its ``str()`` reads like a transport
+            # failure ("status: 200") while the real cause is the body's error code.
+            api_error = _slack_response_payload(getattr(e, "response", None)).get("error")
+            logger.error(
+                "[Slack] Failed to edit message %s in channel %s: api_error=%s: %s",
+                message_id, chat_id, api_error or "none", e, exc_info=True)
             return SendResult(success=False, error=str(e))
 
     async def delete_message(self, chat_id: str, message_id: str) -> bool:

tests/gateway/test_empty_sentinel_rewrite_copy.py

@@ -39,6 +39,8 @@ async def test_empty_sentinel_rewrite_uses_the_shared_explanation_with_the_model
 
 @pytest.mark.asyncio
 async def test_response_ready_log_includes_the_session_key(caplog):
+    """Per-conversation latency aggregation needs the session key on the completion line:
+    ``chat=`` alone merges every Slack thread of one channel (#111931)."""
     runner = _Runner()
     source = SimpleNamespace(chat_id="C1", platform=SimpleNamespace(value="slack"))
 
@@ -53,21 +55,3 @@ async def test_response_ready_log_includes_the_session_key(caplog):
     response_log = next(record.getMessage() for record in caplog.records
                         if record.getMessage().startswith("response ready:"))
     assert "session=slack:C1:thread-123" in response_log
-
-
-@pytest.mark.asyncio
-async def test_response_ready_log_uses_a_safe_session_fallback(caplog):
-    runner = _Runner()
-    source = SimpleNamespace(chat_id="C1", platform=SimpleNamespace(value="slack"))
-
-    with caplog.at_level("INFO", logger="gateway.run"):
-        await runner._hmwa_shape_agent_response(
-            {"final_response": "done", "messages": [], "api_calls": 1},
-            source, history=[], session_entry=SimpleNamespace(session_id="s"),
-            session_key=None, _quick_key=None, run_generation=0,
-            _run_start_session_id="s", _platform_name="slack", _msg_start_time=0.0,
-        )
-
-    response_log = next(record.getMessage() for record in caplog.records
-                        if record.getMessage().startswith("response ready:"))
-    assert "session=unknown" in response_log

tests/gateway/test_slack_send_retry.py

@@ -114,6 +114,8 @@ class TestSlackSendRetryable:
 
     @pytest.mark.asyncio
     async def test_edit_api_failure_logs_the_slack_error_code(self, caplog):
+        """HTTP 200 + ok=false must name the body error code, not read as a transport failure
+        (#111931)."""
         adapter = _make_adapter()
         error = _slack_api_error(200)
         error.response.data = {"ok": False, "error": "message_not_found"}