feat(merge_requests): reconfirm original commit context

apps/labfox/lib/features/merge_requests/data/merge_requests_repository.dart

@@ -1,6 +1,15 @@
 import 'package:gitlab_api/gitlab_api.dart';
 import 'package:gitlab_models/gitlab_models.dart';
 
+import '../../commits/data/history_repository.dart';
+
+/// A read aggregate of existing DTOs, without inferred original coordinates.
+typedef MrOriginalCommitContext = ({
+  MergeRequest mergeRequest,
+  Commit commit,
+  List<CommitDiffFile> files,
+});
+
 /// Loads a project's merge requests and single merge requests.
 class MergeRequestsRepository {
   MergeRequestsRepository(this._client);
@@ -116,6 +125,110 @@ class MergeRequestsRepository {
     return result;
   }
 
+  /// Combines literal original commit reads through one captured account.
+  /// Reconfirms MR origin and membership after fetching the target commit/diff.
+  /// Null means the caller's origin expired. Sequential reads are not atomic;
+  /// callers recheck currency and original coordinates before private writes.
+  Future<MrOriginalCommitContext?> originalCommitContext({
+    required int projectId,
+    required int iid,
+    required int mergeRequestId,
+    required String commitId,
+    required bool Function() isCurrent,
+  }) async {
+    if (projectId < 1) throw ArgumentError.value(projectId, 'projectId');
+    if (iid < 1) throw ArgumentError.value(iid, 'iid');
+    if (mergeRequestId < 1) {
+      throw ArgumentError.value(mergeRequestId, 'mergeRequestId');
+    }
+    if (commitId.length != 40 || RegExp(r'[^0-9a-f]').hasMatch(commitId)) {
+      throw ArgumentError.value(commitId, 'commitId');
+    }
+    if (!isCurrent()) return null;
+    final history = HistoryRepository(_client);
+    try {
+      final initial = await commitReviewContext(
+        projectId: projectId,
+        iid: iid,
+        mergeRequestId: mergeRequestId,
+        isCurrent: isCurrent,
+      );
+      if (initial == null || !isCurrent()) return null;
+      final members = await commits(
+        projectId: projectId,
+        iid: iid,
+        isCurrent: isCurrent,
+      );
+      if (members == null || !isCurrent()) return null;
+      final member = _selectedMember(members, commitId);
+      final selected = await history.originalCommit(
+        projectId: initial.targetProjectId!,
+        commitId: commitId,
+        isCurrent: isCurrent,
+      );
+      if (selected == null || !isCurrent()) return null;
+      _confirmReportedParents(member, selected);
+      final files = await history.originalCommitDiff(
+        projectId: initial.targetProjectId!,
+        commitId: commitId,
+        isCurrent: isCurrent,
+      );
+      if (files == null || !isCurrent()) return null;
+      final refreshed = await commitReviewContext(
+        projectId: projectId,
+        iid: iid,
+        mergeRequestId: mergeRequestId,
+        isCurrent: isCurrent,
+      );
+      if (refreshed == null || !isCurrent()) return null;
+      if (initial.sourceProjectId != refreshed.sourceProjectId ||
+          initial.targetProjectId != refreshed.targetProjectId ||
+          initial.sourceBranch != refreshed.sourceBranch ||
+          initial.targetBranch != refreshed.targetBranch) {
+        throw const GitLabConflictException(
+          'The MR commit origin changed. Reload before continuing.',
+        );
+      }
+      final confirmedMembers = await commits(
+        projectId: projectId,
+        iid: iid,
+        isCurrent: isCurrent,
+      );
+      if (confirmedMembers == null || !isCurrent()) return null;
+      _confirmReportedParents(
+        _selectedMember(confirmedMembers, commitId),
+        selected,
+      );
+      if (!isCurrent()) return null;
+      return (mergeRequest: refreshed, commit: selected, files: files);
+    } on GitLabException {
+      if (!isCurrent()) return null;
+      rethrow;
+    }
+  }
+
+  static Commit _selectedMember(List<Commit> members, String commitId) {
+    for (final member in members) {
+      if (member.id == commitId) return member;
+    }
+    throw const GitLabConflictException(
+      'The selected commit is no longer in the MR. Reload before continuing.',
+    );
+  }
+
+  static void _confirmReportedParents(Commit member, Commit selected) {
+    final reported = member.parentIds, authoritative = selected.parentIds;
+    if (reported == null || authoritative == null) return;
+    if (reported.length != authoritative.length ||
+        Iterable<int>.generate(
+          reported.length,
+        ).any((i) => reported[i] != authoritative[i])) {
+      throw const GitLabServerException(
+        'Inconsistent original commit parents.',
+      );
+    }
+  }
+
   Future<MergeRequest> get({required int projectId, required int iid}) {
     return _client.mergeRequests.get(projectId, iid: iid);
   }