[DRAFT] chore(tests): enable core_deps presubmit check

[daniel-sanche]

This PR re-enabled the core_deps check on pre-submit, with a few changes.

This test triggers the core_deps_from_source nox session, which will run the unit tests against the unreleased copy of the library present in the codebase

Unlike the previous test:

• we take advantage of the new sharding system
• this one only targets handwritten libraries, not all packages in the repo
• tests are trigged for all handwritten libraries when any core dependency is changed (auth, api_core, proto_plus, etc), or if the test config changed

This change also modified noxfiles, to better support preview packages, and avoid downloading over the network when possible

[gemini-code-assist]

Code Review

This pull request introduces a new core_deps_from_source test session across multiple packages, updating CI scripts to trigger downstream testing of handwritten packages when core dependencies or CI configurations change. Feedback on these changes highlights the need to type-validate loaded JSON metadata and normalize path strings in ci/get_package_shards.py to prevent potential runtime errors and cross-platform issues. Additionally, the fragile while loops used to locate the packages directory in several package noxfile.py files should be replaced with a more robust generator expression to ensure compatibility with preview packages.