feat: add size field and builder pattern to Resource record (#331)

The size field helps hosts display file sizes and estimate context window usage.

- Add size parameter to Resource record for tracking raw content size in bytes
- Implement Builder pattern for Resource construction with validation
- Add deprecated constructor for backwards compatibility
- Add tests for builder pattern and field validation

Resolves #330

Signed-off-by: Christian Tzolov <christian.tzolov@broadcom.com>