First agent handoff blocked by process-wide message limit

# First agent handoff blocked by process-wide message limit

## Problem

In VS Code Insiders, a task session completed its work but could not report back to its coordinator. Its first `send_message` attempt failed with:

> Refusing to send more than 50 messages from server tools in this process.

The session log records only one `send_message` attempt in that task session. The completed work was preserved, but the automatic coordinator handoff failed.

## Observed sequence

1. Delegate work to a separate agent session.
2. Complete and validate the task.
3. Attempt to send the completion report to the coordinator using `send_message`.
4. The first attempt is rejected with the error above.

The activity that exhausted the reported process-wide allowance and its reset behavior have not been established, so these are observation steps rather than a deterministic reproduction.

## Expected behavior

Normal coordinator handoffs should work. If a protective messaging limit is reached, the error should explain its scope, current usage, and recovery path rather than leaving a task session unable to deliver its result.