Include merge_commit_sha in the minimal pull request result

pull_request_read (method get) and list_pull_requests return a trimmed
MinimalPullRequest built by convertToMinimalPullRequest, which had no
merge_commit_sha field, so a merged pull request gave no way to reach the commit
the merge produced without a second call (#3235).

Add MergeCommitSHA to MinimalPullRequest and populate it, but only once the PR
is merged. An open PR's merge_commit_sha is GitHub's ephemeral test-merge ref
(refs/pull/N/merge), not a commit on the base branch, so gating on the merged
signal (merged_at present, or merged) avoids handing that back as if it were the
merge commit. Add merge_commit_sha to listPullRequestsItemFieldEnum so
list_pull_requests can select it, and regenerate that tool's snapshot.

Add a converter unit test asserting the field is emitted for a merged PR and
withheld for an open or closed-unmerged one.

Author: thejdubb02

pkg/github/__toolsnaps__/list_pull_requests.snap

@@ -29,6 +29,7 @@
             "state",
             "draft",
             "merged",
+            "merge_commit_sha",
             "mergeable_state",
             "html_url",
             "user",

pkg/github/minimal_types.go

@@ -45,7 +45,7 @@ var listIssuesItemFieldEnum = []any{
 // MinimalPullRequest. The body field is the heaviest, so omitting it is the main
 // lever for shrinking large result sets.
 var listPullRequestsItemFieldEnum = []any{
-	"number", "title", "body", "state", "draft", "merged", "mergeable_state",
+	"number", "title", "body", "state", "draft", "merged", "merge_commit_sha", "mergeable_state",
 	"html_url", "user", "labels", "assignees", "requested_reviewers", "merged_by",
 	"head", "base", "additions", "deletions", "changed_files", "commits",
 	"comments", "created_at", "updated_at", "closed_at", "merged_at", "milestone",
@@ -717,6 +717,7 @@ type MinimalPullRequest struct {
 	State              string           `json:"state"`
 	Draft              bool             `json:"draft"`
 	Merged             bool             `json:"merged"`
+	MergeCommitSHA     string           `json:"merge_commit_sha,omitempty"`
 	MergeableState     string           `json:"mergeable_state,omitempty"`
 	HTMLURL            string           `json:"html_url"`
 	User               *MinimalUser     `json:"user,omitempty"`
@@ -1111,6 +1112,11 @@ func convertToMinimalPullRequest(pr *github.PullRequest) MinimalPullRequest {
 	if pr.MergedAt != nil {
 		m.MergedAt = pr.MergedAt.Format(time.RFC3339)
 	}
+	// merge_commit_sha is a real base-branch commit only once merged; an open PR's
+	// is an ephemeral test-merge ref, so omit it there (#3235).
+	if pr.GetMerged() || pr.MergedAt != nil {
+		m.MergeCommitSHA = pr.GetMergeCommitSHA()
+	}
 
 	for _, label := range pr.Labels {
 		if label != nil {

pkg/github/pullrequests_test.go

@@ -180,6 +180,50 @@ func Test_GetPullRequest(t *testing.T) {
 	}
 }
 
+// Test_convertToMinimalPullRequestMergeCommitSHA pins that merge_commit_sha is
+// emitted only once a PR is merged, not for an open PR's ephemeral test-merge
+// ref (#3235).
+func Test_convertToMinimalPullRequestMergeCommitSHA(t *testing.T) {
+	const sha = "f1e2d3c4b5a6978012345678901234567890abcd"
+	tests := []struct {
+		name string
+		pr   *github.PullRequest
+		want string
+	}{
+		{
+			name: "merged PR exposes the merge commit",
+			pr: &github.PullRequest{
+				State:          github.Ptr("closed"),
+				Merged:         github.Ptr(true),
+				MergedAt:       &github.Timestamp{Time: time.Now()},
+				MergeCommitSHA: github.Ptr(sha),
+			},
+			want: sha,
+		},
+		{
+			name: "open PR withholds the ephemeral test-merge SHA",
+			pr: &github.PullRequest{
+				State:          github.Ptr("open"),
+				MergeCommitSHA: github.Ptr(sha),
+			},
+			want: "",
+		},
+		{
+			name: "closed-unmerged PR withholds any test-merge SHA",
+			pr: &github.PullRequest{
+				State:          github.Ptr("closed"),
+				MergeCommitSHA: github.Ptr(sha),
+			},
+			want: "",
+		},
+	}
+	for _, tc := range tests {
+		t.Run(tc.name, func(t *testing.T) {
+			assert.Equal(t, tc.want, convertToMinimalPullRequest(tc.pr).MergeCommitSHA)
+		})
+	}
+}
+
 func Test_UpdatePullRequest(t *testing.T) {
 	// Verify tool definition once
 	serverTool := UpdatePullRequest(translations.NullTranslationHelper)