Fix API not persisting pull request unit config when has_pull_requests is not set (#36718)

The `PATCH /api/v1/repos/{owner}/{repo}` endpoint silently ignores pull
request config fields (like `default_delete_branch_after_merge`,
`allow_squash_merge`, etc.) unless `has_pull_requests: true` is also
included in the request body. This is because the entire PR unit config
block was gated behind `if opts.HasPullRequests != nil`.

This PR restructures the logic so that PR config options are applied
whenever the pull request unit already exists on the repo, without
requiring `has_pull_requests` to be explicitly set. A new unit is only
created when `has_pull_requests: true` is explicitly sent.

Fixes https://github.com/go-gitea/gitea/issues/36466
Co-authored-by: Claude Opus 4.6 <noreply@anthropic.com>
Co-authored-by: wxiaoguang <wxiaoguang@gmail.com>
Co-authored-by: Giteabot <teabot@gitea.io>

tests/integration/api_repo_edit_test.go

@@ -418,5 +418,20 @@ func TestAPIRepoEdit(t *testing.T) {
 		req = NewRequestWithJSON(t, "PATCH", fmt.Sprintf("/api/v1/repos/%s/%s", user2.Name, repo1.Name), &repoEditOption).
 			AddTokenAuth(token4)
 		MakeRequest(t, req, http.StatusForbidden)
+
+		// Test updating pull request settings without setting has_pull_requests
+		repo1 = unittest.AssertExistsAndLoadBean(t, &repo_model.Repository{ID: 1})
+		url = fmt.Sprintf("/api/v1/repos/%s/%s", user2.Name, repo1.Name)
+		req = NewRequestWithJSON(t, "PATCH", url, &api.EditRepoOption{
+			DefaultDeleteBranchAfterMerge: &bTrue,
+		}).AddTokenAuth(token2)
+		resp = MakeRequest(t, req, http.StatusOK)
+		DecodeJSON(t, resp, &repo)
+		assert.True(t, repo.DefaultDeleteBranchAfterMerge)
+		// reset
+		req = NewRequestWithJSON(t, "PATCH", url, &api.EditRepoOption{
+			DefaultDeleteBranchAfterMerge: &bFalse,
+		}).AddTokenAuth(token2)
+		_ = MakeRequest(t, req, http.StatusOK)
 	})
 }