Add file() directive support for modularizing manifest values

[jeffreyaven]

Summary

This PR adds support for file() directives in manifest property values, enabling users to modularize large manifest values (such as IAM policy statements) into separate JSON or YAML files.

Key Changes

• New error variant: Added FileIncludeError to ManifestError enum for file resolution failures
• File directive parsing: Implemented parse_file_directive() to extract file paths from file(path) syntax with flexible whitespace handling
• Recursive resolution: Added resolve_file_directives() to recursively traverse and resolve file() directives in YAML/JSON value trees (strings, sequences, and mappings)
• File loading: Implemented load_file_contents() to read and parse referenced files with support for:
  • JSON files (.json extension)
  • YAML files (.yml, .yaml extensions)
  • Automatic format detection for files without recognized extensions (tries JSON first, falls back to YAML)
  • Nested file directive resolution within loaded content
• Manifest integration: Added resolve_manifest_file_directives() to process all globals and resource properties in a manifest
• Load-time resolution: Modified Manifest::load_from_file() to automatically resolve file directives relative to <stack_dir>/resources/ directory
• Comprehensive test suite: Added 13 tests covering:
  • Directive parsing edge cases
  • JSON object and array resolution
  • YAML file resolution
  • Nested directives in sequences and mappings
  • Subdirectory paths
  • Error handling for missing files
  • Non-directive string preservation
  • Template variable preservation
  • End-to-end manifest loading with file directives
  • Nested file directive resolution

Implementation Details

• File paths in file() directives are resolved relative to the resources/ subdirectory under the manifest's parent directory
• The resolution process is recursive, allowing included files to themselves contain file() directives
• Non-directive strings and template variables (e.g., {{ stack_name }}) are left untouched
• Error messages include both the original file path and the resolved full path for debugging

https://claude.ai/code/session_01FikwxWpZdvhtNbECVWJiqK