This commit fixes all identified functionality bugs and adds comprehensive
test coverage to prevent regressions.

## Bugs Fixed:

**Fix #1: Exit code extraction no longer contaminates output buffer**
- Modified execute_command to save buffer before extracting exit code
- Created async _extract_exit_code_async method that doesn't pollute output
- Exit code extraction now uses separate buffer that gets discarded

**Fix sdevare-nv#2: Race conditions prevented with proper locking**
- Added async lock to execute_command method
- Prevents concurrent command execution on same session
- Ensures output from different commands doesn't get mixed

**Fix sdevare-nv#3: NameError in create_session error handling fixed**
- Initialize master_fd and process variables before try block
- Prevents NameError if exception occurs before PTY creation
- Also added process cleanup in error handler

**Fix sdevare-nv#4: Robust prompt detection with custom marker**
- Replaced brittle regex patterns ([$#>]$) with unique marker
- Set custom PS1 with PROMPT_MARKER after shell initialization
- Commands outputting $, #, or > no longer cause false timeouts
- Added _clean_output method to remove markers from user-visible output

**Fix sdevare-nv#5: Process state validation added**
- Check if shell process is still alive before executing commands
- Prevents hangs when process has crashed or been killed externally
- Added same check to send_input method

**Fix sdevare-nv#6: Buffer clearing timing issues resolved**
- Read and discard stale output before clearing buffers
- Prevents output from previous commands contaminating current command
- Fixed rapid successive command execution

**Fix sdevare-nv#7: Thread-safe singleton pattern implemented**
- Added async lock for get_session_manager_async
- Created separate sync version for backward compatibility
- Prevents race conditions in manager initialization

**Fix sdevare-nv#8: Complete resource cleanup on errors**
- Kill and wait for process in error handler
- Close file descriptors even if process creation fails
- No more zombie processes or fd leaks

**Fix sdevare-nv#9: Blocking sleep replaced with async sleep**
- Changed time.sleep to await asyncio.sleep in _extract_exit_code_async
- Prevents blocking the event loop
- Improved async performance

## Additional Improvements:

- Enhanced ANSI escape code handling in exit code extraction
- Added support for CSI sequences with ? character
- Improved regex to handle terminal control sequences
- Better exit code parsing for all edge cases

## Test Coverage:

Added comprehensive test suite (test_terminus_bugfixes.py) with 12 tests:
- 7 tests for specific bug fixes
- 5 tests for previously uncovered scenarios:
  * Concurrent command execution
  * Rapid successive commands
  * Environment persistence
  * Exit code accuracy
  * Multi-line output handling

All 12 tests pass successfully.

## Tested Scenarios:

✅ Exit code extraction doesn't contaminate subsequent commands
✅ Concurrent commands are properly serialized
✅ Error handling doesn't raise NameError
✅ Commands with prompt-like output ($, #, >) don't timeout
✅ Dead process detection works correctly
✅ Rapid successive commands don't mix output
✅ Resource cleanup is complete even on errors
✅ Environment variables persist across commands
✅ Exit codes (0, 1, custom) are captured accurately
✅ Multi-line command output is preserved

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>