feat(ai): add partial JSON parsing for streaming tool calls

- Added partial-json package for parsing incomplete JSON during streaming
- Tool call arguments now contain partially parsed JSON during toolcall_delta events
- Enables progressive UI updates (e.g., showing file paths before content is complete)
- Arguments are always valid objects (minimum empty {}), never undefined
- Full validation still occurs at toolcall_end when arguments are complete
- Updated all providers (Anthropic, OpenAI Completions/Responses) to use parseStreamingJson
- Added comprehensive documentation and examples in README
- Added test to verify arguments are always defined during streaming

packages/ai/test/generate.test.ts

@@ -95,6 +95,12 @@ async function handleToolCall<TApi extends Api>(model: Model<TApi>, options?: Op
 			if (toolCall.type === "toolCall") {
 				expect(toolCall.name).toBe("calculator");
 				accumulatedToolArgs += event.delta;
+				// Check that we have a parsed arguments object during streaming
+				expect(toolCall.arguments).toBeDefined();
+				expect(typeof toolCall.arguments).toBe("object");
+				// The arguments should be partially populated as we stream
+				// At minimum it should be an empty object, never undefined
+				expect(toolCall.arguments).not.toBeNull();
 			}
 		}
 		if (event.type === "toolcall_end") {