fix(web-search): handle xAI Responses API format in Grok provider

The xAI /v1/responses API returns content in a structured format with
typed output blocks (type: 'message') containing typed content blocks
(type: 'output_text') and url_citation annotations. The previous code
only checked output[0].content[0].text without filtering by type,
which could miss content in responses with multiple output entries.

Changes:
- Update GrokSearchResponse type to include annotations on content blocks
- Filter output blocks by type='message' and content by type='output_text'
- Extract url_citation annotations as fallback citations when top-level
  citations array is empty
- Deduplicate annotation-derived citation URLs
- Update tests for the new structured return type

Closes #13520

src/agents/tools/web-search.test.ts

@@ -145,21 +145,66 @@ describe("web_search grok config resolution", () => {
 });
 
 describe("web_search grok response parsing", () => {
-  it("extracts content from Responses API output blocks", () => {
-    expect(
-      extractGrokContent({
-        output: [
-          {
-            content: [{ text: "hello from output" }],
-          },
-        ],
-      }),
-    ).toBe("hello from output");
+  it("extracts content from Responses API message blocks", () => {
+    const result = extractGrokContent({
+      output: [
+        {
+          type: "message",
+          content: [{ type: "output_text", text: "hello from output" }],
+        },
+      ],
+    });
+    expect(result.text).toBe("hello from output");
+    expect(result.annotationCitations).toEqual([]);
+  });
+
+  it("extracts url_citation annotations from content blocks", () => {
+    const result = extractGrokContent({
+      output: [
+        {
+          type: "message",
+          content: [
+            {
+              type: "output_text",
+              text: "hello with citations",
+              annotations: [
+                {
+                  type: "url_citation",
+                  url: "https://example.com/a",
+                  start_index: 0,
+                  end_index: 5,
+                },
+                {
+                  type: "url_citation",
+                  url: "https://example.com/b",
+                  start_index: 6,
+                  end_index: 10,
+                },
+                {
+                  type: "url_citation",
+                  url: "https://example.com/a",
+                  start_index: 11,
+                  end_index: 15,
+                }, // duplicate
+              ],
+            },
+          ],
+        },
+      ],
+    });
+    expect(result.text).toBe("hello with citations");
+    expect(result.annotationCitations).toEqual(["https://example.com/a", "https://example.com/b"]);
   });
 
   it("falls back to deprecated output_text", () => {
-    expect(extractGrokContent({ output_text: "hello from output_text" })).toBe(
-      "hello from output_text",
-    );
+    const result = extractGrokContent({ output_text: "hello from output_text" });
+    expect(result.text).toBe("hello from output_text");
+    expect(result.annotationCitations).toEqual([]);
+  });
+
+  it("returns undefined text when no content found", () => {
+    const result = extractGrokContent({});
+    expect(result.text).toBeUndefined();
+    expect(result.annotationCitations).toEqual([]);
   });
 });