test(spanner): upgrade Spanner sample instance fixtures to session scope

[sakthivelmanii]

Previously, standard and multi-region Spanner instance fixtures in conftest.py were module-scoped, meaning a brand-new Spanner instance was created and destroyed for every single test file, adding massive execution time (~2 minutes per file) in CI/CD runs.

This change upgrades the instance and config fixtures to session scope. To prevent ScopeMismatch exceptions, the dependencies on module-scoped sample_name are replaced with a generic shared-samples label. This allows the test suite to reuse a single shared Spanner instance, dramatically optimizing test execution times.

Thank you for opening a Pull Request! Before submitting your PR, there are a few things you can do to make sure it goes smoothly:

• Make sure to open an issue as a bug/issue before writing your code! That way we can discuss the change, evaluate designs, and agree on the general idea
• Ensure the tests and linter pass
• Code coverage does not decrease (if any source code was changed)
• Appropriate docs were updated (if necessary)

Fixes #<issue_number_goes_here> 🦕

[gemini-code-assist]

Code Review

This pull request removes the sample_name fixture across multiple test files and centralizes instance labeling in conftest.py by using a hardcoded "shared-samples" value. Additionally, several fixtures in conftest.py have been upgraded from module to session scope. A potential issue was identified regarding a ScopeMismatch error, as the newly session-scoped sample_instance fixture depends on other fixtures that might still be module-scoped.

[gemini-code-assist]

The sample_instance fixture has been upgraded to session scope, but it depends on spanner_client and cleanup_old_instances. If these fixtures are still module-scoped (which was the previous scope for most fixtures in this file), pytest will raise a ScopeMismatch error. Please ensure that spanner_client, cleanup_old_instances, and any other dependencies of session-scoped fixtures are also upgraded to session scope.

[olavloite]

spanner_client and cleanup_old_instances were already session scope, so this should not cause any problems.

[olavloite]

Won't this cause race conditions if multiple integration test runs are started in parallel? They will now use the same instance, and potentially try to use the same database name.

[sakthivelmanii]

I fixed it. created a new unique db name