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/src/providers/anthropic.ts

@@ -5,6 +5,7 @@ import type {
 	MessageParam,
 } from "@anthropic-ai/sdk/resources/messages.js";
 import { AssistantMessageEventStream } from "../event-stream.js";
+import { parseStreamingJson } from "../json-parse.js";
 import { calculateCost } from "../models.js";
 import type {
 	Api,
@@ -124,6 +125,7 @@ export const streamAnthropic: StreamFunction<"anthropic-messages"> = (
 						const block = blocks[index];
 						if (block && block.type === "toolCall") {
 							block.partialJson += event.delta.partial_json;
+							block.arguments = parseStreamingJson(block.partialJson);
 							stream.push({
 								type: "toolcall_delta",
 								contentIndex: index,