refactor(mcp): remove redundant type field from Content implementations (#27)

- The type field and associated methods were redundant in Content implementations (TextContent, ImageContent, EmbeddedResource)
  as the type information is already handled by Jackson's polymorphic type handling via @JsonSubTypes annotation.
- Add default implementation that returns the appropriate type string based on the implementing class (text, image, or resource)
- Added comprehensive unit tests for McpSchema to validate serialization/deserialization behavior of all schema components.
- Add deserialization tests for all content types
- Add json-unit-assertj for flexible JSON testing -  ignores array order and extra array items, reducing test brittleness while
maintaining functional validation.

Resolves #26
Resolve https://github.com/spring-projects/spring-ai/issues/2350



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