Add support for parallel processing of alias groups in WP-CLI.

Currently, when running a command against an alias group (e.g., @all or a custom group like @both), WP-CLI processes each alias sequentially. This change adds support for parallel execution via the WP_CLI_ALIAS_GROUPS_PARALLEL environment variable, significantly improving execution speed when running commands across multiple WordPress installations.

Changes Made

• Add WP_CLI_ALIAS_GROUPS_PARALLEL environment variable support
• Modify the run_alias_group() method in Runner.php to check the environment variable
• Add feature tests to validate parallel execution works correctly
• Add documentation and examples for the environment variable
• Ensure backward compatibility (default behavior remains sequential)
• Pass code review with no issues
• Pass security scan with no vulnerabilities
• Fix Gherkin test style to use 'And' instead of 'When'
• Fix security issues: escape $script_path and $alias values
• Eliminate code duplication by precomputing command components
• Document non-deterministic output behavior in parallel mode
• Merge changes from main branch including quiet mode support and type annotations

Implementation Details

The implementation uses the WP_CLI_ALIAS_GROUPS_PARALLEL environment variable:

1. When set to a truthy value (e.g., 1), spawns all alias processes concurrently before waiting for results
2. When not set (default), maintains the original sequential behavior
3. Works with all types of aliases (local paths, SSH, docker, etc.)
4. Environment variable is naturally inherited by child processes
5. All shell arguments are properly escaped to prevent injection attacks
6. Command components are precomputed once to avoid duplication
7. Integrates with main branch's quiet mode exclusion logic
8. Uses proper PHPDoc annotations and type casts from main branch

Note: In parallel mode, output from multiple processes will be interleaved and non-deterministic. This is expected behavior when running commands concurrently.

Usage Examples

# Run command against multiple aliases in parallel
WP_CLI_ALIAS_GROUPS_PARALLEL=1 wp @both option get home

# Run command against all registered aliases in parallel
WP_CLI_ALIAS_GROUPS_PARALLEL=1 wp @all plugin list

# Works with quiet mode
WP_CLI_ALIAS_GROUPS_PARALLEL=1 wp @all core version --quiet

Performance Benefits

This change provides significant performance improvements when:

• Running commands across multiple remote installations over SSH
• Executing I/O-bound operations
• Processing commands that take significant time to complete
• Working with installations on different machines or containers

Backward Compatibility

The default behavior remains unchanged - aliases are processed sequentially unless the WP_CLI_ALIAS_GROUPS_PARALLEL environment variable is set. This ensures no breaking changes for existing workflows.

Security Summary

✅ All shell arguments are properly escaped using escapeshellarg()
✅ No new security vulnerabilities introduced
✅ Code review completed with no issues
✅ CodeQL security scan completed with no findings

• Fixes Add switch to allow aliases to be processed in parallel #6128

• Fixes Add switch to allow aliases to be processed in parallel #6128

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