A client called `run_in_transaction` be stuck if Spanner is down

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### Summary of the issue

**Context**
When the Spanner is unavailable. If we call [`run_in_transaction`](https://github.com/googleapis/google-cloud-python/blob/main/packages/google-cloud-spanner/google/cloud/spanner_v1/database.py#L1015), the Spanner client will stuck forever.

We believe the [get_session](https://github.com/googleapis/google-cloud-python/blob/main/packages/google-cloud-spanner/google/cloud/spanner_v1/database.py#L1064) should in the try catch block. Otherwise, the `self._local.transaction_running = True` will be forever True.

**Expected Behavior:**
The client will retry when the Spanner DB is back.

**Actual Behavior:**
The client stuck forever.

### API client name and version

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### Reproduction steps: code

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### Reproduction steps: supporting files

- Try to create a session when Spanner is unavailable.
- The client will be stuck forever.

### Reproduction steps: actual results

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### Reproduction steps: expected results

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### OS & version + platform

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### Python environment

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### Python dependencies

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### Additional context

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Start in packages/google-cloud-spanner/google/cloud/spanner_v1/database.py at run_in_transaction and its get_session call. Trace the transaction-running state when session creation fails while Spanner is unavailable, then add coverage for that failure path. Done means the client does not remain stuck and can retry when Spanner becomes available.

Scritto dal modello di indicizzazione a partire dal testo della issue.