Agent Host session remains unusable after referenced MCP server is removed

## Summary

An existing Agent Host session can become permanently unusable when it references an MCP server that has since been removed from the user's MCP configuration. The session continues to fail on the stale MCP reference even though that server no longer appears in the MCP list.

## Steps to reproduce

1. Start an Agent Host session with an MCP server configured.
2. Remove that MCP server from the MCP configuration/list.
3. Return to the existing session.
4. Send another message, or use `/btw`.

## Actual behavior

The session fails with an error referring to the removed MCP server. Every subsequent interaction fails the same way, including `/btw`, so the session cannot be used anymore.

## Expected behavior

The session should reconcile its MCP configuration with the currently available servers and remain usable. A removed MCP server should not permanently block unrelated prompts or `/btw` requests.

## Additional context

A screenshot and Agent Host debug-log export were provided with the original report. The failure persists even though the referenced MCP server is no longer present in the MCP list.

## Duplicate search

No matching open or closed microsoft/vscode issue was found for a session remaining blocked by a stale MCP reference after the server is removed. #336219 concerns a tool catalog changing before invocation, but does not describe an existing session becoming permanently unusable after removing an MCP server.

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Written with AI assistance.