fix: enable cancellation of slash commands and background processes

[akhil29]

Summary

Enables proper cancellation when users press Escape during slash command execution, including termination of background processes spawned by shell commands. Addresses issue #6066 where slash commands did not respect the top-level cancellation signal.

Key Changes

• AbortSignal propagation: Added signal parameter to slash command processor chain
• Background process termination: Implemented killAllBackgroundProcesses() with cross-platform support
• Global PID tracking: Added tracking for background processes across async boundaries
• Signal integration: Connected cancellation hooks throughout command execution flow
• Comprehensive tests: Added test coverage for new functionality

Technical Implementation

Core Cancellation Flow

1. User presses Escape → useGeminiStream.cancelOngoingRequest()
2. Calls killAllBackgroundProcesses() to terminate spawned processes
3. AbortSignal propagates through handleSlashCommand → createCommandContext
4. Commands receive signal and can respond to cancellation appropriately

Background Process Management

• Cross-platform termination: Uses process groups (SIGTERM/SIGKILL) on Unix, taskkill on Windows
• Global PID tracking: Maintains Set of background PIDs for cleanup on cancellation
• Graceful shutdown: SIGTERM followed by SIGKILL after timeout

Code Quality Improvements

• Refactored to useCallback: Addressed memoization code smell from PR feedback
• Signal parameter pattern: Changed from creating dummy signals to accepting signal as parameter
• App-level AbortController: Added for programmatic command lifecycle management

Test Coverage

• ✅ Background process termination: Tests Unix/Windows process killing, error handling, PID tracking
• ✅ Integration tests: Verifies cancellation flow in useGeminiStream
• ✅ Signal propagation: Tests AbortSignal parameter passing through command chain
• ✅ Cross-platform support: Tests both Unix and Windows process termination methods

Fixes

• Background processes now properly terminate when cancellation occurs (addresses #6066)
• Shell commands like /tmp/long_counter.sh & can be cancelled with Escape
• Proper signal propagation prevents zombie processes and resource leaks
• useCallback pattern eliminates memoization code smell from previous implementation

Fixes #6066