Summary: Add debug backtrace on exit ✅

Issue: When WP_CLI::error() or WP_CLI::halt() was called (especially from plugins/themes during early initialization), there was no way to identify where the call originated from, even with --debug enabled. This made debugging extremely difficult.

Solution: When --debug is enabled, display the file/line and backtrace when exit methods are called.

Changes Made

Core Implementation

• ✅ Added debug_backtrace_on_exit() private method to capture and format backtraces
• ✅ Modified WP_CLI::error() to call backtrace method before exiting
• ✅ Modified WP_CLI::halt() to call backtrace method before exiting
• ✅ Optimized loop performance (cached count() result)
• ✅ Updated PHPDoc comments for both methods
• ✅ Fixed code review feedback: Removed unnecessary isset check for 'function', added isset for 'type', changed to pre-increment
• ✅ Updated to use null coalescing operator for cleaner syntax (PHP 7.2+ requirement)

Testing

• ✅ Added 3 comprehensive test scenarios in features/framework.feature
• ✅ Manual testing with multiple edge cases
• ✅ Verified backward compatibility (no changes without --debug)

Files Changed

• php/class-wp-cli.php (+73 lines)
• features/framework.feature (+71 lines)

Example Output

Before (without --debug) - UNCHANGED

$ wp --require=plugin.php
Error: Plugin initialization failed.

After (with --debug) - NEW FEATURE

$ wp --require=plugin.php --debug
Error: Plugin initialization failed.
Debug (bootstrap): Script called exit from: /path/to/plugin.php:42 (0.006s)
Debug (bootstrap): Backtrace:
  #0 my_function() called at [/path/to/plugin.php:42]
  #1 __construct() called at [/path/to/plugin.php:15]
  #2 require_once() called at [/home/runner/work/wp-cli/php/utils.php:165]
  ...

Testing Results ✅

All manual tests passed:

• Simple function calls - Shows exact file and line ✅
• Nested function calls - Shows complete call stack ✅
• Class methods - Shows class and method names with -> operator ✅
• Static methods - Shows class and method names with :: operator ✅
• $exit = false - No backtrace (correct) ✅
• Without --debug - No backtrace (backward compatible) ✅
• Complex plugin scenario - Identifies problematic code ✅

Benefits

1. Dramatically reduces debugging time - No more searching through all plugins
2. Identifies problematic code - Shows exact file:line where error occurred
3. Provides context - Call stack shows how the code was reached
4. Non-intrusive - Only appears with --debug flag
5. Handles all scenarios - Functions, nested calls, classes, static methods, etc.

Ready for review and CI testing!

• Fixes --debug should display info on what called the exit #6009

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