feat: Spanner Mock Server#16041

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bhatt4982 wants to merge 7 commits intomainfrom

Commits on Mar 9, 2026

feat: Add a new `google-cloud-spanner-mockserver` package providing an in-memory mock Spanner server for testing.
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test: add unit tests and configure nox to run them.
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chore: Add `__init__.py` files to `google-cloud-spanner-mockserver/tests`
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chore: migrate linting and formatting to ruff and add mypy type checking to the noxfile.
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refactor: Replace pyproject.toml with setup.py, add mypy configuration, and update noxfile.
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feat: Add Python 3.10-3.14 constraint files, introduce a `prerelease_deps` nox session for testing, and refine standard test dependencies.
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feat: Add stateful session retrieval, listing, and deletion, and transaction rollback to mock Spanner.
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