Change test case terminology and create sample for snapshot isolation

[prathmeshcc]

This PR addresses the following:

• Changes the terminology for client default options commented for transaction unit test.
• Creates a Sample for Snapshot Isolation

[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 C++ client library by introducing a practical example for managing transaction isolation levels and improving the clarity of related documentation and test comments. These changes aim to provide developers with better guidance and understanding of how to effectively configure and utilize different isolation levels within their applications.

Highlights

• New Isolation Level Example: A new C++ sample (samples.cc) has been added to demonstrate how to configure transaction isolation levels, both at the client-level and by overriding them at the transaction-level.
• Documentation Update: The spanner-main.dox documentation has been updated to include a new page (spanner-isolation-level-example) that describes the support for isolation levels and links to the new sample.
• Terminology Refinement: Comments in transaction.h and transaction_test.cc have been updated to use more precise terminology regarding transaction isolation levels, specifically clarifying "snapshot transactions" to "repeatable read transactions" and "client default options" to "default options".

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[gemini-code-assist]

Code Review

This pull request introduces a new sample for setting transaction isolation levels in Spanner, along with corresponding documentation updates. It also refines some terminology in comments within the transaction-related tests and headers for better clarity. The changes are well-structured. I've suggested a couple of improvements to the new sample code to demonstrate best practices by using SQL parameters instead of hardcoded values.

[gemini-code-assist]

For improved security and to prevent SQL injection vulnerabilities, it's a best practice to use query parameters instead of embedding literal values directly into the SQL string. This also improves readability and maintainability.

        auto sql = spanner::SqlStatement(
            "SELECT AlbumTitle from Albums WHERE SingerId = @SingerId and AlbumId = @AlbumId",
            {{"SingerId", spanner::Value(1)}, {"AlbumId", spanner::Value(1)}});

[gemini-code-assist]

Similar to the SELECT statement, it's recommended to use parameters for DML statements to prevent SQL injection and improve code clarity.

        auto update_sql = spanner::SqlStatement(
            "UPDATE Albums "
            "SET AlbumTitle = @AlbumTitle "
            "WHERE SingerId = @SingerId and AlbumId = @AlbumId",
            {{"AlbumTitle", spanner::Value("New Album Title")},
             {"SingerId", spanner::Value(1)},
             {"AlbumId", spanner::Value(1)}});

[codecov]

Codecov Report

❌ Patch coverage is 10.34483% with 26 lines in your changes missing coverage. Please review.
✅ Project coverage is 92.95%. Comparing base (3959c49) to head (3b360f6).

Files with missing lines	Patch %	Lines
google/cloud/spanner/samples/samples.cc	10.34%	26 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #15895      +/-   ##
==========================================
- Coverage   92.95%   92.95%   -0.01%     
==========================================
  Files        2460     2460              
  Lines      228641   228670      +29     
==========================================
+ Hits       212544   212550       +6     
- Misses      16097    16120      +23     

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[prathmeshcc]

Branch was deleted/recreated; opening a fresh PR for these terminology changes in #15900