This pull request introduces agent card validation capabilities to the project, updates documentation to reflect these new features, and improves CLI usability and output formatting. The most important changes are grouped below by theme.

Agent Card Validation Feature:

• Added a new module validation.py that provides utilities to validate agent cards from URLs or local files, returning structured results and warnings about best practices. This includes robust error handling for HTTP, file, and JSON errors.
• Updated documentation (AGENTS.md, CONTRIBUTING.md, README.md) to describe the new just validate and handler validate commands, including usage examples for validating agent cards from both URLs and files. [1] [2] [3] [4] [5]

CLI and Output Improvements:

• Switched the CLI implementation from click to rich-click for enhanced formatting and user experience, and updated dependencies accordingly. [1] [2] [3]
• Expanded CLI documentation to include global options (verbose, debug, help) and new output options for commands, such as JSON output for card, send, and validate.

Client and Output Formatting Enhancements:

• Added a ParsedResponse dataclass and a parse_response utility to client.py for extracting and handling text content from A2A responses. [1] [2]
• Improved JSON output formatting in common/printing.py by switching from syntax highlighting to structural highlighting using rich.json, resulting in clearer and more readable JSON panels. [1] [2]

Project Metadata:

• Bumped the project version to 0.1.8 in pyproject.toml to reflect these new features and improvements.