docs: add reconnection error scenarios per review feedback

Add NOT_FOUND and FAILED_PRECONDITION error cases to reconnection
docs in specification.md and streaming-and-async.md per
gemini-code-assist suggestions.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: James Chang

docs/specification.md

@@ -248,7 +248,7 @@ The operation MUST establish a bidirectional streaming connection. The stream MU
 
 **Reconnection:**
 
-A client may reconnect to an ongoing task by specifying a `taskId` with empty message parts. If the task is in a terminal state, the agent returns the task and half-closes. If the task is active, the agent sends the current state and begins sending updates.
+A client may reconnect to an ongoing task by specifying a `taskId` with empty message parts. If the task is in a terminal state, the agent returns the task and half-closes. If the task is active, the agent sends the current state and begins sending updates. If no task exists with the specified ID, the agent SHOULD return a `NOT_FOUND` error. If another client already has an active stream for the task and the agent does not support multiple connections, the agent MAY return a `FAILED_PRECONDITION` error.
 
 #### 3.1.4. Get Task
 

docs/topics/streaming-and-async.md

@@ -64,7 +64,7 @@ For tasks that require human-in-the-loop interactions — such as multi-step app
 
 - **Stream Termination:** The agent MUST half-close the connection when the task reaches a terminal state (`COMPLETED`, `FAILED`, `CANCELED`, `REJECTED`).
 
-- **Reconnection:** A client can reconnect to an ongoing task by calling `SendLiveMessage` with a `taskId` and empty message parts. If the task is active, the agent sends the current state and resumes streaming updates.
+- **Reconnection:** A client can reconnect to an ongoing task by calling `SendLiveMessage` with a `taskId` and empty message parts. If the task is active, the agent sends the current state and resumes streaming updates. If the task is already in a terminal state, the agent returns the final task details and half-closes the stream. If the task ID is not found, or if another stream is already active and the agent doesn't support multiplexing, an error (`NOT_FOUND` or `FAILED_PRECONDITION`) is returned.
 
 ### When to Use Bidirectional Streaming