Flaky spanner test `client::tests::transaction_timeout_respected` timeouts on `run`

Error Log:

```text
failures:
---- client::tests::transaction_timeout_respected stdout ----
thread 'client::tests::transaction_timeout_respected' (81097) panicked at src/spanner/src/client.rs:2020:16:
Transaction should have succeeded: Error { kind: Timeout, source: Some(Status { code: Cancelled, message: "Timeout expired", source: Some(tonic::transport::Error(Transport, TimeoutExpired(()))) }) }
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace
failures:
client::tests::transaction_timeout_respected
test result: FAILED. 1341 passed; 1 failed; 1 ignored; 0 measured; 0 filtered out; finished in 6.39s
error: test failed, to rerun pass `-p google-cloud-spanner --lib`
```

https://pantheon.corp.google.com/cloud-build/builds;region=us-central1/3d85359f-681e-4216-bfdf-e0d014c9d4e3?project=rust-sdk-testing&pli=1&e=SqlStudioMysqlLaunch::SqlStudioMysqlEnabled&mods=allow_workbench_image_override

Start by rerunning client::tests::transaction_timeout_respected in the google-cloud-spanner package and inspect the test around src/spanner/src/client.rs:2020. Compare the timeout failure with successful runs to identify the source of the flakiness. Done means the test reliably passes without weakening the timeout assertion, and the package test suite remains green.

Written by the indexing model from the issue text.