feat(spanner): samples to set transaction options

[skuruppu]

Adds samples that show how to set the isolation level and read lock mode at the Spanner client level and at a transaction level. If it's set at the client level, all RW transactions will set this value. The transaction-level setting allows users to override the client-level setting or to set the values for specific transactions.

[snippet-bot]

Here is the summary of changes.

You are about to add 2 region tags.

• spanner/spanner_isolation_level.rb:15, tag spanner_isolation_level
• spanner/spanner_read_lock_mode.rb:15, tag spanner_read_lock_mode

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[gemini-code-assist]

Code Review

This pull request introduces two new Ruby Spanner examples: one demonstrating how to configure isolation levels and another for read lock modes. Both examples illustrate setting these options at the client level and overriding them at the transaction level. Corresponding RSpec tests have been added for these new functionalities. The review comments correctly identify a critical issue in both test files where the database schema is redundantly created, which would cause the tests to fail. This can be resolved by removing the duplicate db.update calls in the test setup.