feat: add Image type for image-specific file handling
This PR adds a new `cog.Image` type that extends `cog.Path` to provide
image-specific handling in model inputs and outputs.
The Image type works identically to Path but produces OpenAPI schemas
with `format: "uri"` and `x-cog-type: "image"`, following the same
pattern as streaming outputs that use `x-cog-array-type: "iterator"`.
This allows UIs to provide image-specific widgets while correctly
identifying the value as a URI.
Changes:
- Add Image class in cog/types.py with Pydantic v1 and v2 support
- Export Image from cog package
- Update AST-based OpenAPI schema generation to handle Image type
- Add comprehensive unit and integration tests
- Update documentation with Image type examples
