Test all packages

[parthea]

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Fixes #<issue_number_goes_here> 🦕

[gemini-code-assist]

Code Review

This pull request introduces a new batching script ci/get_batches.py to group packages for parallelized testing, updates ci/run_conditional_tests.sh to support testing specific directories and force-testing all packages, and lowers the test coverage threshold in packages/django-google-spanner/noxfile.py. Feedback highlights that the coverage threshold should not be disabled (set to 0), the shebang in ci/get_batches.py needs to be moved to the first line to execute correctly, and a potential double-slash issue in the glob expansion of ci/run_conditional_tests.sh should be resolved.

[gemini-code-assist]

Lowering the coverage threshold to 0 disables coverage enforcement for this package. If this was done temporarily for testing or debugging, please revert it to the original value of 75 to ensure code coverage standards are maintained.

Suggested change

	"--cov-fail-under=0",
	"--cov-fail-under=75",

[gemini-code-assist]

The shebang line #!/usr/bin/env python3 is currently placed on line 14, after the copyright header. For the operating system to recognize the shebang and execute the script using the Python interpreter when run directly (e.g., ./ci/get_batches.py), the shebang must be the very first line of the file (line 1), preceding even the copyright comments.

[gemini-code-assist]

Using ${subdir} directly when it has a trailing slash (e.g., packages/) results in a double slash in the expanded glob (e.g., packages//google-cloud-storage/). Using ${subdir%/} strips any trailing slash and prevents double slashes.

Suggested change

	loop_dirs=("${subdir}"/*/)
	loop_dirs=("${subdir%/}"/*/)