Fix get reviewers' bug (#32415)

This PR rewrites `GetReviewer` function and move it to service layer.

Reviewers should not be watchers, so that this PR removed all watchers
from reviewers. When the repository is under an organization, the pull
request unit read permission will be checked to resolve the bug of
#32394

Fix #32394

models/organization/team_repo_test.go

@@ -0,0 +1,31 @@
+// Copyright 2024 The Gitea Authors. All rights reserved.
+// SPDX-License-Identifier: MIT
+
+package organization_test
+
+import (
+	"testing"
+
+	"code.gitea.io/gitea/models/db"
+	"code.gitea.io/gitea/models/organization"
+	"code.gitea.io/gitea/models/perm"
+	"code.gitea.io/gitea/models/repo"
+	"code.gitea.io/gitea/models/unit"
+	"code.gitea.io/gitea/models/unittest"
+
+	"github.com/stretchr/testify/assert"
+)
+
+func TestGetTeamsWithAccessToRepoUnit(t *testing.T) {
+	assert.NoError(t, unittest.PrepareTestDatabase())
+
+	org41 := unittest.AssertExistsAndLoadBean(t, &organization.Organization{ID: 41})
+	repo61 := unittest.AssertExistsAndLoadBean(t, &repo.Repository{ID: 61})
+
+	teams, err := organization.GetTeamsWithAccessToRepoUnit(db.DefaultContext, org41.ID, repo61.ID, perm.AccessModeRead, unit.TypePullRequests)
+	assert.NoError(t, err)
+	if assert.Len(t, teams, 2) {
+		assert.EqualValues(t, 21, teams[0].ID)
+		assert.EqualValues(t, 22, teams[1].ID)
+	}
+}