Fix Spanner write mutation coder serialization

[NA-V10]

Summary

This change explicitly sets spannerMutationCoder on the SCollection[Mutation] before applying the Beam Spanner write transform.

Previously, the write path relied on Beam/Kryo fallback serialization, which could lead to UnsupportedOperationException when serializing Mutation / MutationGroup.

Changes Made

• Materialized spannerMutationCoder using CoderMaterializer.beam
• Applied the coder to the input SCollection[Mutation]
• Updated the write path to use data.setCoder(coder).applyInternal(transform)

Why

A spannerMutationCoder already exists in CoderInstances, but it was not being explicitly used in SpannerWrite. This change ensures that Beam uses the intended serializer for Mutation objects during writes.

[NA-V10]

Thanks for reporting this issue.

I updated the Spanner write path to explicitly use spannerMutationCoder before applying the Beam write transform. Since the coder already existed in CoderInstances, the fix was mainly wiring it into SpannerWrite so Beam does not fall back to Kryo serialization for Mutation objects.