fix(repo): preserve transfer recipient collaboration (#39042)

Remove temporary recipient access after a transfer ends while preserving
existing collaboration.

---------

Co-authored-by: silverwind <me@silverwind.io>
Co-authored-by: wxiaoguang <wxiaoguang@gmail.com>

services/repository/collaboration_test.go

@@ -11,6 +11,7 @@ import (
 	"gitea.dev/models/perm"
 	access_model "gitea.dev/models/perm/access"
 	repo_model "gitea.dev/models/repo"
+	"gitea.dev/models/unit"
 	"gitea.dev/models/unittest"
 	user_model "gitea.dev/models/user"
 
@@ -98,3 +99,22 @@ func TestRepository_DeleteCollaborationRemovesSubscriptionsAndStopwatches(t *tes
 	assert.NoError(t, err)
 	assert.False(t, hasStopwatch)
 }
+
+func TestRepository_DeleteCollaborationPreservesWatchWithPublicAccess(t *testing.T) {
+	assert.NoError(t, unittest.PrepareTestDatabase())
+
+	ctx := t.Context()
+	user := unittest.AssertExistsAndLoadBean(t, &user_model.User{ID: 5})
+	assert.NoError(t, repo_model.UpdateRepoUnitPublicAccess(ctx, &repo_model.RepoUnit{
+		RepoID: 2, Type: unit.TypeIssues, EveryoneAccessMode: perm.AccessModeRead,
+	}))
+	repo := unittest.AssertExistsAndLoadBean(t, &repo_model.Repository{ID: 2})
+	assert.NoError(t, repo.LoadOwner(ctx))
+	assert.NoError(t, AddOrUpdateCollaborator(ctx, repo, user, perm.AccessModeRead))
+	assert.NoError(t, repo_model.WatchRepoAuto(ctx, user, repo, true))
+
+	assert.NoError(t, DeleteCollaboration(ctx, repo, user))
+	watch, err := repo_model.GetWatch(ctx, user.ID, repo.ID)
+	assert.NoError(t, err)
+	assert.True(t, repo_model.IsWatchModeWatching(watch.Mode))
+}