Extends the Content discriminated union with a fifth variant for
inline image payloads. Image carries the raw bytes (captured at
user-input time so the message snapshot is self-contained and
survives source-file deletion), the IANA media type for the
provider's image part, and the original path for logging.
HasImages() lets providers decide whether to fall back to a
text-only representation; providers that don't know about
ContentImage will simply skip those blocks via TextContent().
Add custom MarshalJSON/UnmarshalJSON on Content using string type discriminant
("text", "tool_call", "tool_result", "thinking"). Add json tags to Message.
vikingowl