chore: add missing nox sessions and polish dependencies in django-google-spanner

[chalmerlowe]

Description

This PR implements missing nox sessions and refactors dependency management inside packages/django-google-spanner/noxfile.py to align with google-cloud standard practices.

---

Key Changes

1. Constants Architecture

All dependencies and version definitions have been concentrated at the top of noxfile.py in nestled, unpinned constants:

• UNIT_TEST_STANDARD_DEPENDENCIES: Standard testing frameworks/runners (mock, pytest, pytest-cov, coverage).
• UNIT_TEST_EXTERNAL_DEPENDENCIES: Telemetry and setuptools helpers (unpinned to delegate versioning strictly to setup.py and lower-bounds in constraints.txt).
• UNIT_TEST_DEPENDENCIES: Connector library requirements (django~=5.2 and sqlparse==0.3.1 pinned to protect SQL compiler string assertions).
• UNIT_TEST_MOCKSERVER_DEPENDENCIES: Specialized mockserver overrides (django~=5.2, google-cloud-spanner>=3.55.0, sqlparse>=0.4.4).

NOTES:

• core_deps_from_source & prerelease_deps: Standard dependencies are first installed normally from PyPI, and then target core packages are cleanly overwritten with local source paths/pre-releases using pip's native --ignore-installed --no-deps flags.
• mypy: Implemented and configured to skip gracefully with a standard TODO issue link (issue/17047) indicating that typehints will be added on a separate task.
• format: Integrated standard Ruff formatting and import sorting.

Fixes: #17047

[gemini-code-assist]

Code Review

This pull request refactors noxfile.py to modularize unit test dependencies into reusable lists and implements the previously skipped core_deps_from_source and prerelease_deps test sessions, parametrizing them over protobuf implementations. The feedback highlights three key improvements: first, core_deps_from_source should check for the existence of local dependency directories before installation to prevent directory-not-found errors; second, the dynamic Django version replacement should be made more robust by checking if a dependency starts with "django" rather than relying on a hardcoded string replacement; and third, Python version comparisons should use integer tuples instead of string comparisons to avoid fragile evaluations.