Support beam:logical_type:micros_instant:v1 in SpannerIo.

[claudevdm]

This adds support for the micros instant logical type in spanner IO reads and writes.

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[github-actions]

Assigning reviewers:

R: @shunping for label python.
R: @Abacn for label java.
R: @nielm for label spanner.

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[claudevdm]

/gemini review

[gemini-code-assist]

Code Review

This pull request adds support for microsecond-precision timestamps (MicrosInstant logical type) in SpannerIO. The changes correctly handle the conversion between java.time.Instant and Spanner's Timestamp for both writing and reading data. The integration tests have also been updated to cover this new functionality.

My review focuses on improving code maintainability by reducing duplication and enhancing the completeness of the integration tests. I've identified a few areas where helper methods could be introduced to centralize conversion logic, and a couple of test cases where assertions could be made more robust.

[Abacn]

LGTM after dedup conversion logics suggested by AI code review

[nielm]

Please also implement the gemini code assist changes...

[claudevdm]

@nielm can you please take another look?

[codecov]

Codecov Report

❌ Patch coverage is 76.92308% with 12 lines in your changes missing coverage. Please review.
✅ Project coverage is 40.34%. Comparing base (c8d7ca0) to head (c170f82).
⚠️ Report is 31 commits behind head on master.

Files with missing lines	Patch %	Lines
.../apache/beam/sdk/io/gcp/spanner/MutationUtils.java	74.28%	6 Missing and 3 partials ⚠️
...rg/apache/beam/sdk/io/gcp/spanner/StructUtils.java	82.35%	0 Missing and 3 partials ⚠️

Additional details and impacted files

@@            Coverage Diff            @@
##             master   #36840   +/-   ##
=========================================
  Coverage     40.33%   40.34%           
- Complexity     3456     3469   +13     
=========================================
  Files          1225     1225           
  Lines        187817   187859   +42     
  Branches       3586     3597   +11     
=========================================
+ Hits          75762    75793   +31     
- Misses       108665   108668    +3     
- Partials       3390     3398    +8     

Flag	Coverage Δ
java	70.56% <76.92%> (+<0.01%)	⬆️

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