fix(gitstore): adjust garbage collection to run after push operation

- Updated `maybeRunGC` to accept `repoDir` instead of `repo`.
- Moved garbage collection trigger to occur after the push step for improved reliability.
- Added a test to validate the sequence of push and GC operations.

Closes: #3373

internal/store/gitstore.go

@@ -858,7 +858,6 @@ func (s *GitTokenStore) commitAndPushLocked(message string, relPaths ...string)
 	} else if errRewrite := s.rewriteHeadAsSingleCommit(repo, headRef.Name(), commitHash, message, signature); errRewrite != nil {
 		return errRewrite
 	}
-	s.maybeRunGC(repo)
 	pushOpts := &git.PushOptions{Auth: s.gitAuth(), Force: true}
 	if s.branch != "" {
 		pushOpts.RefSpecs = []config.RefSpec{config.RefSpec("refs/heads/" + s.branch + ":refs/heads/" + s.branch)}
@@ -874,6 +873,7 @@ func (s *GitTokenStore) commitAndPushLocked(message string, relPaths ...string)
 		}
 		return fmt.Errorf("git token store: push: %w", err)
 	}
+	s.maybeRunGC(repoDir)
 	return nil
 }
 
@@ -907,13 +907,18 @@ func (s *GitTokenStore) rewriteHeadAsSingleCommit(repo *git.Repository, branch p
 	return nil
 }
 
-func (s *GitTokenStore) maybeRunGC(repo *git.Repository) {
+func (s *GitTokenStore) maybeRunGC(repoDir string) {
 	now := time.Now()
 	if now.Sub(s.lastGC) < gcInterval {
 		return
 	}
 	s.lastGC = now
 
+	repo, err := git.PlainOpen(repoDir)
+	if err != nil {
+		return
+	}
+
 	pruneOpts := git.PruneOptions{
 		OnlyObjectsOlderThan: now,
 		Handler:              repo.DeleteObject,