Make merge_commit_sha nullable in GitHub server

Fixes modelcontextprotocol#227.

Author: sd2k

src/github/schemas.ts

@@ -281,7 +281,7 @@ export const GitHubPullRequestSchema = z.object({
   updated_at: z.string(),
   closed_at: z.string().nullable(),
   merged_at: z.string().nullable(),
-  merge_commit_sha: z.string(),
+  merge_commit_sha: z.string().nullable(),
   assignee: GitHubIssueAssigneeSchema.nullable(),
   assignees: z.array(GitHubIssueAssigneeSchema),
   head: GitHubPullRequestHeadSchema,
@@ -375,4 +375,4 @@ export type CreateIssueOptions = z.infer<typeof CreateIssueOptionsSchema>;
 export type CreatePullRequestOptions = z.infer<typeof CreatePullRequestOptionsSchema>;
 export type CreateBranchOptions = z.infer<typeof CreateBranchOptionsSchema>;
 export type GitHubCreateUpdateFileResponse = z.infer<typeof GitHubCreateUpdateFileResponseSchema>;
-export type GitHubSearchResponse = z.infer<typeof GitHubSearchResponseSchema>;
+export type GitHubSearchResponse = z.infer<typeof GitHubSearchResponseSchema>;