refactor: consolidate content annotations into Annotations (#242)

Updates all Content types to represent annotations as nested objects
instead of directly on the content itself.

- Refactor TextContent, ImageContent, and EmbeddedResource to use Annotations object instead of separate audience/priority fields
- Add backward-compatible deprecated methods for audience() and priority() accessors
- Add parameterized tests for message annotations across different message types (success, error, debug)
- Test both text and image content annotations with proper priority and audience validation

This brings the implementation in line with the content types as modeled by the specification:
https://github.com/modelcontextprotocol/modelcontextprotocol/blob/c87a0da6d8c2436d56a6398023c80b0562224454/schema/2025-03-26/schema.json#L1970-L1973