test(Bigtable): configure per-operation timeout and totalTimeoutMillis for MutateRow in test proxy

[JoeWang1127]

In the Bigtable test proxy, MutateRow previously passed timeoutMillis unconditionally without configuring retrySettings.totalTimeoutMillis.

When MutateRow has retries enabled (or when DEADLINE_EXCEEDED is configured as retryable), failing to set totalTimeoutMillis causes GAX to fall back to the default total timeout (60s) and retry upon receiving a deadline timeout. This caused tests like TestMutateRow_Generic_DeadlineExceeded to send unexpected retry attempts, crashing the mock test server.

This change:

1. Conditionally checks if perOperationTimeout is set to avoid passing null values into RetrySettings, preventing ValidationException.
2. Sets both timeoutMillis and retrySettings.totalTimeoutMillis so that:
   • On branches where retries are disabled (e.g. main), timeoutMillis ensures the call times out after the requested deadline.
   • On branches where retries are enabled, totalTimeoutMillis ensures GAX bounds the total operation time and stops retrying once the deadline is exceeded.

Fixes googleapis/librarian#7405