GitDagBundle: promoting a SHA-pinned tracking_ref can fail with 'reference is not a tree' if local storage isn't cleared

### Description

`GitDagBundle` supports pinning `tracking_ref` to a full commit SHA (see #69735). Rolling back to an older SHA works reliably after a config change + Dag processor restart, because the objects are already present in local storage. **Promoting** to a new SHA that was created/pushed after the bundle's local storage was first populated can fail.

### Root cause

In [`GitDagBundle._initialize`](https://github.com/apache/airflow/blob/6af6a6a6bfe216618b0af4171bd5f1b10c83d14e/providers/git/src/airflow/providers/git/bundles/git.py#L153-L239):

- The working clone lives at a stable path (`self.repo_path`, e.g. `.../tracking_repo` when no separate `version` is set), not a fresh path per `tracking_ref` value.
- `_clone_repo_if_required()` only clones if that path doesn't already exist. On a restart where the bundle's storage directory has survived (the default — `dag_bundle_storage_path` defaults to `/tmp/airflow/dag_bundles`), the existing working clone is reopened, not re-fetched.
- Only the **bare mirror** is fetched inside `_clone_bare_repo_if_required` → `_fetch_bare_repo()`. The working clone (`tracking_repo`) is only fetched inside `refresh()`, which runs *after* the `self.repo.git.checkout(self.tracking_ref)` call at line 215 for the no-`version` code path.
- If the new SHA (or a newly created tag) isn't yet an object in the stale working clone, `checkout` raises `GitCommandError: fatal: reference is not a tree: ` (or `pathspec '' did not match` for a new tag).

### Repro

1. Configure a `GitDagBundle` with `tracking_ref` pinned to commit A. Let the Dag processor initialize the bundle (populates `tracking_repo`).
2. Push a new commit B to the source repo, then update the bundle config's `tracking_ref` to commit B's SHA.
3. Restart the Dag processor without clearing the bundle's local storage path.
4. `initialize()` raises `GitCommandError: fatal: reference is not a tree: `.

Rolling back to commit A (already present locally) after this failure succeeds normally. A fresh storage path (new pod, or manually deleted bundle directory) also succeeds for promotion, since `_clone_repo_if_required()` then performs a real clone against the already-updated bare mirror.

### Suggested fix

In `_initialize`, fetch the working clone (or check that the target ref is present and fetch if not) before calling `self.repo.git.checkout(self.tracking_ref)`, mirroring what `refresh()` already does for the branch/tag tracking case.

### Related

- Uncovered during review of #69735, which documents `tracking_ref`'s SHA support and scopes its restart-behavior docs/tests to this limitation until it's fixed here.

Start in providers/git/src/airflow/providers/git/bundles/git.py, focusing on GitDagBundle._initialize and the existing refresh() path. Reproduce promotion from an older pinned SHA to a newly pushed SHA with the bundle storage retained, then add regression coverage for initialization. Done means a new SHA or tag can be checked out after restart without clearing local storage.

Written by the indexing model from the issue text.