feat: Add default PR branch update style setting (#37410)

Adds repository-level settings for pull request branch updates so admins
can choose the default update method and disable merge or rebase
updates.

<img width="1025" height="158"
src="https://github.com/user-attachments/assets/d030973b-0ddd-4035-b04f-145c445084d7"
/>

---------

Co-authored-by: OpenAI Codex (GPT-5) <codex@openai.com>
Co-authored-by: silverwind <me@silverwind.io>
Co-authored-by: Claude (Opus 4.7) <noreply@anthropic.com>
Co-authored-by: wxiaoguang <wxiaoguang@gmail.com>

models/repo/pull_request_default_test.go

@@ -8,6 +8,7 @@ import (
 
 	"code.gitea.io/gitea/models/unit"
 	"code.gitea.io/gitea/models/unittest"
+	"code.gitea.io/gitea/modules/util"
 
 	"github.com/stretchr/testify/assert"
 )
@@ -30,3 +31,82 @@ func TestDefaultTargetBranchSelection(t *testing.T) {
 	repo.Units = nil
 	assert.Equal(t, "branch2", repo.GetPullRequestTargetBranch(ctx))
 }
+
+func TestPullRequestConfigFromDB(t *testing.T) {
+	cases := []struct {
+		// name describes the row shape under test; the comments capture why each row matters.
+		name              string
+		json              string
+		wantMergeUpdate   bool
+		wantRebaseUpdate  bool
+		wantDefaultStyle  UpdateStyle
+		wantValidatesPass bool
+	}{
+		{
+			// Empty object exercises the all-defaults path (e.g. fresh repos created via low-level paths).
+			name: "defaults", json: "{}",
+			wantMergeUpdate: true, wantRebaseUpdate: true,
+			wantDefaultStyle: UpdateStyleMerge, wantValidatesPass: true,
+		},
+		{
+			// Realistic upgrade case: pre-PR JSON lacks the new fields and has AllowRebaseUpdate=false.
+			// Historical setting must be preserved while new fields take safe defaults.
+			name:            "legacy without new fields",
+			json:            `{"AllowMerge":true,"AllowRebase":true,"AllowRebaseMerge":true,"AllowSquash":true,"AllowRebaseUpdate":false}`,
+			wantMergeUpdate: true, wantRebaseUpdate: false,
+			wantDefaultStyle: UpdateStyleMerge, wantValidatesPass: true,
+		},
+		{
+			// Partially-migrated row with explicit empty string must normalize so ValidateUpdateSettings passes.
+			name: "empty default style", json: `{"DefaultUpdateStyle":""}`,
+			wantMergeUpdate: true, wantRebaseUpdate: true,
+			wantDefaultStyle: UpdateStyleMerge, wantValidatesPass: true,
+		},
+	}
+	for _, tc := range cases {
+		t.Run(tc.name, func(t *testing.T) {
+			cfg := new(PullRequestsConfig)
+			assert.NoError(t, cfg.FromDB([]byte(tc.json)))
+			assert.Equal(t, tc.wantMergeUpdate, cfg.AllowMergeUpdate)
+			assert.Equal(t, tc.wantRebaseUpdate, cfg.AllowRebaseUpdate)
+			assert.Equal(t, tc.wantDefaultStyle, cfg.DefaultUpdateStyle)
+			if tc.wantValidatesPass {
+				assert.NoError(t, cfg.ValidateUpdateSettings())
+			}
+		})
+	}
+}
+
+func TestPullRequestConfigValidateUpdateSettingsInvalidArgument(t *testing.T) {
+	cases := []struct {
+		name string
+		cfg  PullRequestsConfig
+	}{
+		{
+			name: "invalid default style",
+			cfg: PullRequestsConfig{
+				AllowMergeUpdate:   true,
+				AllowRebaseUpdate:  true,
+				DefaultUpdateStyle: "invalid",
+			},
+		},
+		{
+			name: "no update style enabled",
+			cfg: PullRequestsConfig{
+				DefaultUpdateStyle: UpdateStyleMerge,
+			},
+		},
+		{
+			name: "default update style disabled",
+			cfg: PullRequestsConfig{
+				AllowRebaseUpdate:  true,
+				DefaultUpdateStyle: UpdateStyleMerge,
+			},
+		},
+	}
+	for _, tc := range cases {
+		t.Run(tc.name, func(t *testing.T) {
+			assert.ErrorIs(t, tc.cfg.ValidateUpdateSettings(), util.ErrInvalidArgument)
+		})
+	}
+}