chore(logging): include PR number when detecting existing branch PR (renovatebot#40832)

feat(logging): include PR number when detecting existing branch PR

Include the PR number in debug log messages when:
- An existing open branch PR is found ("Found existing branch PR #123")
- A closed PR already exists ("Closed PR #123 already exists")

This aids debugging by making it easier to correlate log entries with
specific PRs when reviewing Renovate logs.

Co-authored-by: Claude Opus 4.5 <noreply@anthropic.com>

Author: ZackKanter

lib/workers/repository/update/branch/index.spec.ts

@@ -2839,6 +2839,7 @@ describe('workers/repository/update/branch/index', () => {
       scm.branchExists.mockResolvedValueOnce(true);
       platform.getBranchPr.mockResolvedValueOnce(
         partial<Pr>({
+          number: 5,
           sourceBranch: 'old/some-branch',
           state: 'open',
         }),
@@ -2858,7 +2859,7 @@ describe('workers/repository/update/branch/index', () => {
         commitSha: '123test',
       });
 
-      expect(logger.debug).toHaveBeenCalledWith('Found existing branch PR');
+      expect(logger.debug).toHaveBeenCalledWith('Found existing branch PR #5');
 
       expect(logger.debug).toHaveBeenCalledWith(
         'No package files need updating',
@@ -2877,6 +2878,7 @@ describe('workers/repository/update/branch/index', () => {
       scm.branchExists.mockResolvedValueOnce(true);
       platform.getBranchPr.mockResolvedValueOnce(
         partial<Pr>({
+          number: 5,
           sourceBranch: 'old/some-branch',
           state: 'open',
         }),
@@ -2897,7 +2899,7 @@ describe('workers/repository/update/branch/index', () => {
         commitSha: null,
       });
 
-      expect(logger.debug).toHaveBeenCalledWith('Found existing branch PR');
+      expect(logger.debug).toHaveBeenCalledWith('Found existing branch PR #5');
       expect(logger.debug).not.toHaveBeenCalledWith(
         'No package files need updating',
       );

lib/workers/repository/update/branch/index.ts

@@ -193,7 +193,7 @@ export async function processBranch(
     } else if (!branchPr && existingPr && !dependencyDashboardCheck) {
       logger.debug(
         { prTitle: config.prTitle },
-        'Closed PR already exists. Skipping branch.',
+        `Closed PR #${existingPr.number} already exists. Skipping branch.`,
       );
       await handleClosedPr(config, existingPr);
       return {
@@ -302,7 +302,7 @@ export async function processBranch(
         config.baseBranch,
       );
       if (branchPr) {
-        logger.debug('Found existing branch PR');
+        logger.debug(`Found existing branch PR #${branchPr.number}`);
         if (branchPr.state !== 'open') {
           logger.debug(
             'PR has been closed or merged since this run started - aborting',