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

test-partial-json.js

@@ -0,0 +1,52 @@
+import { parseStreamingJson } from "./packages/ai/dist/json-parse.js";
+
+// Test cases for partial JSON parsing
+const testCases = [
+    // Complete JSON
+    { input: '{"name":"test","value":42}', expected: {name: "test", value: 42} },
+
+    // Partial JSON - incomplete object
+    { input: '{"name":"test","val', expected: {name: "test"} },
+    { input: '{"name":"test"', expected: {name: "test"} },
+    { input: '{"name":', expected: {} },
+    { input: '{"', expected: {} },
+    { input: '{', expected: {} },
+
+    // Partial JSON - incomplete array
+    { input: '{"items":[1,2,3', expected: {items: [1, 2, 3]} },
+    { input: '{"items":[1,2,', expected: {items: [1, 2]} },
+    { input: '{"items":[', expected: {items: []} },
+
+    // Partial JSON - incomplete string
+    { input: '{"message":"Hello wor', expected: {message: "Hello wor"} },
+
+    // Empty or invalid
+    { input: '', expected: {} },
+    { input: null, expected: {} },
+    { input: undefined, expected: {} },
+
+    // Complex nested partial
+    { input: '{"user":{"name":"John","age":30,"address":{"city":"New Y', expected: {user: {name: "John", age: 30, address: {city: "New Y"}}} },
+];
+
+console.log("Testing parseStreamingJson...\n");
+
+let passed = 0;
+let failed = 0;
+
+for (const test of testCases) {
+    const result = parseStreamingJson(test.input);
+    const success = JSON.stringify(result) === JSON.stringify(test.expected);
+
+    if (success) {
+        console.log(`✅ PASS: "${test.input || '(empty)'}" -> ${JSON.stringify(result)}`);
+        passed++;
+    } else {
+        console.log(`❌ FAIL: "${test.input || '(empty)'}"`);
+        console.log(`   Expected: ${JSON.stringify(test.expected)}`);
+        console.log(`   Got:      ${JSON.stringify(result)}`);
+        failed++;
+    }
+}
+
+console.log(`\n${passed} passed, ${failed} failed`);