Add background process tracking support for Windows in shell tool

[akhil29]

Description

The current implementation of background process tracking in the shell tool has limited support for Windows. While Unix-like systems can track background processes using pgrep and process groups, Windows doesn't have equivalent functionality implemented.

Current Behavior

On Unix/Linux/macOS:

• Background processes are tracked via a temp file using pgrep
• Process groups are used to terminate all child processes
• SIGTERM followed by SIGKILL ensures proper cleanup

On Windows:

• Background process PIDs are not tracked (pgrep not available)
• Only the main process can be terminated with taskkill
• Child processes may become orphaned

Proposed Solution

Implement Windows-specific background process tracking:

1. Use Windows Job Objects or WMI to track child processes
2. Implement a Windows-equivalent of the pgrep functionality
3. Ensure all child processes are terminated when cancellation occurs

Related PR

This issue was identified during the implementation of #7790 which adds cancellation support for slash commands and background processes.

Acceptance Criteria

• [ ] Background processes launched with & are tracked on Windows
• [ ] All child processes are terminated when the parent is cancelled
• [ ] Tests verify Windows-specific functionality
• [ ] Cross-platform behavior is consistent