docs(spanner): add sample for snapshot isolation level

[prathmeshcc]

docs(spanner): add sample for snapshot isolation level

Add a new sample demonstrating how to set and override transaction isolation levels (Repeatable Read vs Serializable). This includes the required documentation snippet tags and integration with the samples binary.

[gemini-code-assist]

Summary of Changes

Hello @prathmeshcc, I'm Gemini Code Assist¹! I'm currently reviewing this pull request and will post my feedback shortly. In the meantime, here's a summary to help you and other reviewers quickly get up to speed!

This pull request enhances the Google Cloud Spanner client library by providing a new sample that illustrates the usage and configuration of transaction isolation levels. Users can now see how to set a default isolation level at the client level and how to override it for specific transactions, offering greater control over transaction behavior. Additionally, minor terminology adjustments have been made in existing code and tests to improve readability.

Highlights

• New Isolation Level Sample: Introduced a new sample demonstrating how to configure and override transaction isolation levels (Repeatable Read vs Serializable) in Google Cloud Spanner.
• Documentation Updates: Added corresponding documentation for the new isolation level sample, including snippet tags and integration into the samples binary.
• Terminology Refinement: Updated terminology within the Transaction class definition and related test cases for improved clarity and consistency.

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[gemini-code-assist]

Code Review

This pull request adds a helpful new sample demonstrating transaction isolation levels. The implementation is well done, but I've found a minor syntax error in one of the new documentation tags that would cause a compilation failure. Please see the specific comment for the fix.

[codecov]

Codecov Report

❌ Patch coverage is 91.17647% with 3 lines in your changes missing coverage. Please review.
✅ Project coverage is 92.96%. Comparing base (b5bad6b) to head (69f202f).
⚠️ Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
google/cloud/spanner/samples/samples.cc	91.17%	3 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #15899      +/-   ##
==========================================
- Coverage   92.96%   92.96%   -0.01%     
==========================================
  Files        2460     2460              
  Lines      228864   228898      +34     
==========================================
+ Hits       212770   212784      +14     
- Misses      16094    16114      +20     

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[devbww]

The argument to SampleBanner() should be the sample tag, which, according to the START/END annotations you added, would be "spanner_isolation_level".