[Feature Request] Support Message Insertion During Task Execution (Full-Duplex Communication)

## Problem Statement

Currently when a user clicks the "Stop" button (the send button transforms into a stop button during task execution):

1. **Frontend stops immediately** - The UI stops responding
2. **Backend keeps running** - The Agent task continues in the background
3. **New messages are lost** - Messages sent during execution are not processed

## Expected Behavior

When user clicks the send/stop button during task execution:

1. **Stop the current task** - Backend should immediately terminate the running task
2. **Insert the user's message** - The new message should be added to the current session
3. **Continue execution** - Agent should resume with the new context (not start a new conversation)

## Use Case

This is essential for real-time human-AI collaboration:
- User can correct AI mistakes mid-execution
- User can provide additional context without waiting
- User can redirect AI focus when it goes off track

## Session Isolation

This feature should only affect the current session:
- Multiple sessions should remain independent
- Stopping one session should not affect others

## Technical Notes

This appears to require changes to:
- Backend `stopChat` API - needs to truly interrupt the running task
- Message handling - needs to queue and process new messages after stop
- Session management - ensure message is inserted into the correct session