Add custom MarshalJSON/UnmarshalJSON on Content using string type discriminant ("text", "tool_call", "tool_result", "thinking"). Add json tags to Message.
internal/message/ — Content discriminated union, Message, Usage, StopReason, Response. 22 tests. internal/stream/ — Stream pull-based iterator interface, Event types, Accumulator (assembles Response from events). 8 tests. internal/provider/ — Provider interface, Request, ToolDefinition, Registry with factory pattern, ProviderError with HTTP status classification. errors.AsType[E] for Go 1.26. 13 tests. 43 tests total, all passing.
