183602 Fix an issue with updating proper job attempt numbers on re-run (#4988)

### 1. Refactored Pull Request Retrieval
- **Updated Handlers:** The logic in `rerun_all_failed_jobs.dart` and `rerun_failed_job.dart` has been updated to consistently use `PrCheckRuns.findPullRequestFor` to resolve the pull request via the commit SHA (`guard.commitSha`), rather than querying by PR number.
- **Fixed Re-run Workflows:** Ensured that the handlers fetch the correct active pull request during re-runs and store the proper retry number/attempt information when invoking `reScheduleTryBuilds`. 

### 2. Removed `pull_request_num` Field and Cache
- **Model Cleanups:** The `pull_request_num` field was entirely removed from the `PrCheckRuns` Firestore model, including its related mappings and integration test matchers (`_pr_check_run.dart`). 
- **Removed Methods:** Deleted the obsolete `findPullRequestCachedForPullRequestNum` lookup method from both `PrCheckRuns` and the `Scheduler` service. Additionally, removed its related caching tests.

### 3. Service/Dependencies Cleanup
- **Removed Unused Services:** Dropped the previously injected but unused `_githubService` from `scheduler.dart`.
- **Code Formatting:** Ran standard Dart formatting rules (`dart format`) to ensure code uniformity in edited files.

### 4. Test Suite Adjustments
- **Updated Mocks & Setup:** `rerun_all_failed_jobs_test.dart` and `rerun_failed_job_test.dart` were updated to properly seed data using `PrCheckRuns.initializeDocument(...)` mirroring production behavior.
- **Updated Assertions:** Test assertions were updated to verify correct PR handling, most notably checking that `reScheduleTryBuilds` receives accurate target configurations (e.g., proper attempt increments like `containsPair(targetA, 2)`). 
- **Removed Deprecated Mocks:** Scrubbed generated files like `mocks.mocks.dart` and testing `data_seeder.dart` to remove references to the deleted fields and getters.

fix: https://github.com/flutter/flutter/issues/183602