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/json-parse.ts

@@ -0,0 +1,28 @@
+import { parse as partialParse } from "partial-json";
+
+/**
+ * Attempts to parse potentially incomplete JSON during streaming.
+ * Always returns a valid object, even if the JSON is incomplete.
+ *
+ * @param partialJson The partial JSON string from streaming
+ * @returns Parsed object or empty object if parsing fails
+ */
+export function parseStreamingJson<T = any>(partialJson: string | undefined): T {
+	if (!partialJson || partialJson.trim() === "") {
+		return {} as T;
+	}
+
+	// Try standard parsing first (fastest for complete JSON)
+	try {
+		return JSON.parse(partialJson) as T;
+	} catch {
+		// Try partial-json for incomplete JSON
+		try {
+			const result = partialParse(partialJson);
+			return (result ?? {}) as T;
+		} catch {
+			// If all parsing fails, return empty object
+			return {} as T;
+		}
+	}
+}