fix github label validation and error handling (#1235)

**github labels fix:**
- allow None for label description field (github api returns null sometimes)
- fixes pydantic validation error when fetching issues with labels

**error handling fix:**
- remove ERROR_HANDLED completed state
- errors now properly fail the flow after sending user-friendly message
- maintains user experience while allowing proper monitoring/alerting

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-authored-by: Claude <noreply@anthropic.com>

Author: zzstoatzz

examples/slackbot/src/slackbot/api.py

@@ -279,12 +279,7 @@ async def handle_message(payload: SlackPayload, db: Database):
                 channel_id=event.channel,
                 thread_ts=thread_ts,
             )
-            # Still return completed so we don't retry
-            return Completed(
-                message="Error during agent execution",
-                name="ERROR_HANDLED",
-                data=dict(error=str(e), user_context=user_context),
-            )
+            raise
         finally:
             try:
                 await mark_thread_completed(db, message_ts)

examples/slackbot/src/slackbot/github/models.py

@@ -18,7 +18,7 @@ class GitHubLabel(BaseModel):
 
     name: str = Field(default="")
     color: str = Field(default="")
-    description: str = Field(default="")
+    description: str | None = Field(default="")
 
 
 class GitHubComment(BaseModel):