[BUG] Input text cleared when @mentioning MCP server to toggle

Bug Description

When typing a message and then using @mention to toggle an MCP server on/off, the input text typed before the @mention is cleared.

Steps to Reproduce

1. Start typing a message in the prompt
2. Type @<mcp-server-name> to toggle it on/off
3. Select the MCP server from the suggestions

Observed Behavior

Input text before the @mention is cleared/lost.

Expected Behavior

Input text should be preserved after toggling the MCP server.

Use Case

User has all MCPs disabled by default to reduce context overhead. When handling a one-off task (e.g., using Linear), they want to type their request and then enable the specific MCP inline with @linear without losing their typed message.

Current Workarounds

• Use /mcp enable <server> before typing the message (less ergonomic)
• Create dedicated subagents per MCP (requires extra setup, less flexible)

Environment

• Platform: macOS
• Version: 2.0.67