add tests for content file

[anaslimem]

Summary

Add tests for TextContent, ImageContent, and AudioContent types in src/content.rs.

Tests Added

Test	Purpose
test_text_content_roundtrip	Verify serialize → deserialize works
test_text_content_omits_optional_fields	Verify skip_serializing_if omits annotations/meta when None
test_text_content_from_string	Verify From<&str> for ContentBlock creates Text variant
test_image_content_roundtrip	Verify serialize → deserialize works
test_image_content_omits_optional_fields	Verify skip_serializing_if omits optional fields when None
test_image_content_with_uri	Verify optional uri field serializes correctly
test_audio_content_roundtrip	Verify serialize → deserialize works
test_audio_content_omits_optional_fields	Verify skip_serializing_if omits annotations/meta when None

Testing

• cargo test - all 37 tests pass
• cargo clippy - no warnings