fix(spanner): preserve call options when applying statement timeout

[akash329d]

Follow-up to #12726. The grpc-gcp affinity routing it (and googleapis/grpc-gcp-java#235) introduced is bypassed when a statement_timeout is set. With multiplexed sessions every data RPC then falls back to the api-config affinity_key: "session" rule and has the same original issue.

Build the configurator's result from the input context instead so the call options survive the merge. The relative timeout is still applied; merge() already requires a GrpcCallContext, so the cast is safe.

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[gemini-code-assist]

Code Review

This pull request modifies AbstractBaseUnitOfWork.java to ensure that existing GrpcCallContext options, such as affinity keys, are preserved when configuring call timeouts. By building from the provided input context instead of creating a default one, the change prevents existing call options from being dropped during the merge process. There were no review comments provided for this pull request, so I have no feedback to provide regarding the review itself.