enhance(pull)!: add per-repository default squash merge commit message (#39816)

Adds a repository setting for the default squash merge commit message
body with GitHub's options `PR_BODY`, `COMMIT_MESSAGES` and `BLANK`,
also settable via `squash_merge_commit_message` in the API. The commit
title stays the pull request title.

1. Breaking: API squash merges without `merge_message_field` now get the
same default message as the web merge box instead of a title-only
commit, which affects clients sending only `Do` like Renovate. An empty
value keeps title-only.
2. Breaking: `[repository.pull-request] DEFAULT_SQUASH_MERGE_MESSAGE`
replaces `POPULATE_SQUASH_COMMENT_WITH_COMMIT_MESSAGES`, which still
works as a deprecated fallback. Repositories store the default when
created or next saved.
3. Deprecated app.ini keys now migrate through one table instead of
per-loader fallbacks, which also fixes the ignored `[mailer] CERT_FILE`
and `KEY_FILE` fallbacks. Deprecation warnings no longer claim a removal
version.

<img width="1891" height="571" alt="grafik"
src="https://github.com/user-attachments/assets/0fdcde13-1f11-4156-b598-d80eb0d04cbe"
/>

---------

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

tests/integration/api_pull_test.go

@@ -283,6 +283,17 @@ func TestAPIMergePull(t *testing.T) {
 			performMerge(t, prDTO.Index, map[string]any{"do": "merge", "delete_branch_after_merge": true})
 			checkBranchExists(t, newBranch, http.StatusNotFound)
 		})
+
+		t.Run("SquashMergePreservesFormMessage", func(t *testing.T) {
+			req := NewRequestWithValues(t, http.MethodPost, fmt.Sprintf("/api/v1/repos/%s/%s/pulls/%d/merge", owner.Name, repo.Name, createTestBranchPR(t, "squash-form-message").Index), map[string]string{
+				"do":                  "squash",
+				"merge_message_field": "custom squash message",
+			}).AddTokenAuth(apiCtx.Token)
+			MakeRequest(t, req, http.StatusOK)
+
+			req = NewRequestf(t, http.MethodGet, "/api/v1/repos/%s/%s/git/commits/%s", owner.Name, repo.Name, repo.DefaultBranch).AddTokenAuth(apiCtx.Token)
+			assert.Contains(t, DecodeJSON(t, MakeRequest(t, req, http.StatusOK), &api.Commit{}).RepoCommit.Message, "\n\ncustom squash message")
+		})
 	})
 }