feat: support audio content

[LucaButBoring]

Adds support for a distinct AudioContent type as defined in the specification. This happens to share a structure with ImageContent, but should probably be distinguished for parity with the specification itself.

Motivation and Context

This PR brings the SDK closer to parity with the written MCP specification.

How Has This Been Tested?

Unit tests.

Breaking Changes

None.

Types of changes

• [ ] Bug fix (non-breaking change which fixes an issue)
• [x] New feature (non-breaking change which adds functionality)
• [ ] Breaking change (fix or feature that would cause existing functionality to change)
• [ ] Documentation update

Checklist

• [x] I have read the MCP Documentation
• [x] My code follows the repository's style guidelines
• [x] New and existing tests pass locally
• [x] I have added appropriate error handling
• [x] I have added or updated documentation as needed

Additional context

[Kludex]

I think there's a version of the supported spec somewhere?

[LucaButBoring]

I think there's a version of the supported spec somewhere?

That would be here - audio content was added in that version, 2025-03-26 (see here, here)

[LucaButBoring]

Also noted in the release notes here.