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

src/github/data/fetcher.ts

@@ -424,7 +424,12 @@ export async function fetchGitHubData({
       if (prResult.repository.pullRequest) {
         const pullRequest = prResult.repository.pullRequest;
         contextData = pullRequest;
-        changedFiles = pullRequest.files.nodes || [];
+        if (pullRequest.files === null) {
+          console.warn(
+            `GitHub did not return the file list for PR #${prNumber} (diff likely too large); proceeding without file-level context`,
+          );
+        }
+        changedFiles = pullRequest.files?.nodes ?? [];
         comments = filterCommentsByActor(
           filterCommentsToTriggerTime(
             pullRequest.comments?.nodes || [],

src/github/data/formatter.ts

@@ -28,7 +28,7 @@ PR Labels: ${formatLabels(prData.labels.nodes)}
 PR Additions: ${prData.additions}
 PR Deletions: ${prData.deletions}
 Total Commits: ${prData.commits.totalCount}
-Changed Files: ${prData.files.nodes.length} files`;
+Changed Files: ${prData.files ? `${prData.files.nodes.length} files` : "unknown (file list unavailable)"}`;
   } else {
     const issueData = contextData as GitHubIssue;
     const sanitizedTitle = sanitizeContent(issueData.title);

src/github/types.ts

@@ -85,9 +85,13 @@ export type GitHubPullRequest = {
       commit: GitHubCommit;
     }>;
   };
+  // GitHub's GraphQL `files` field resolves to null when the PR's diff is too
+  // large for GitHub to compute (very large PRs). `changedFiles` is also
+  // misreported as 0 in that case, so the null must be guarded and treated as
+  // "file list unavailable" rather than "no files changed".
   files: {
     nodes: GitHubFile[];
-  };
+  } | null;
   comments: {
     nodes: GitHubComment[];
   };