fix: handle null files field from GraphQL on very large PRs (#1593)

GitHub's GraphQL API returns files: null (with no errors entry, and
changedFiles misreported as 0) when a PR's diff is too large to compute.
The unguarded pullRequest.files.nodes dereference in the fetcher crashed
the action with 'TypeError: null is not an object', and the formatter had
the same latent crash on prData.files.nodes.length.

Widen the GitHubPullRequest type to files | null so the compiler enforces
guards, degrade gracefully in the fetcher with a warning, and render the
file count as unavailable (not '0 files') in the formatter.

Fixes #1587

test/data-fetcher.test.ts

@@ -1775,6 +1775,49 @@ describe("fetchGitHubData integration with time filtering", () => {
     // Webhook says no body at trigger time — attacker-added GraphQL body must not be used
     expect(result.contextData.body).toBe("");
   });
+
+  it("should not crash when GraphQL returns null files for a very large PR", async () => {
+    // GitHub declines to compute the diff for very large PRs: `files` comes
+    // back as null (with no errors entry) and `changedFiles` is misreported
+    // as 0. The fetch must degrade gracefully instead of throwing.
+    const mockOctokits = {
+      graphql: jest.fn().mockResolvedValue({
+        repository: {
+          pullRequest: {
+            number: 7912,
+            title: "Very large PR",
+            body: "PR body",
+            author: { login: "author" },
+            createdAt: "2024-01-15T10:00:00Z",
+            state: "OPEN",
+            labels: { nodes: [] },
+            comments: { nodes: [] },
+            files: null,
+            reviews: { nodes: [] },
+          },
+        },
+        user: { login: "trigger-user" },
+      }),
+      rest: {
+        pulls: {
+          listFiles: jest.fn().mockResolvedValue({ data: [] }),
+        },
+      },
+    };
+
+    const result = await fetchGitHubData({
+      octokits: mockOctokits as any,
+      repository: "test-owner/test-repo",
+      prNumber: "7912",
+      isPR: true,
+      triggerUsername: "trigger-user",
+      triggerTime: "2024-01-15T12:00:00Z",
+    });
+
+    // No file list is available, so the PR is processed without file-level context.
+    expect(result.changedFiles).toEqual([]);
+    expect(result.changedFilesWithSHA).toEqual([]);
+  });
 });
 
 describe("filterCommentsByActor", () => {