fix: formalize cross-lib public contracts

[kevinmessiaen]

Description

• Added missing public exports to __all__
• Updated upstream lib to import from main entry point
• Added regression tests

Type of Change

• 🥂 Improvement (non-breaking change which improves an existing feature)

Checklist

• I've read the CODE_OF_CONDUCT.md document.
• I've read the CONTRIBUTING.md guide.
• I've written tests for all new methods and classes that I created.

[gemini-code-assist]

Code Review

This pull request refactors imports across the giskard-agents, giskard-checks, and giskard-scan packages to ensure that upstream dependencies are imported strictly from their package roots rather than internal submodules. It also introduces automated tests (test_public_import_surface.py) to enforce this import structure. The feedback recommends explicitly specifying encoding="utf-8" when calling path.read_text() in these new test files to prevent potential UnicodeDecodeError issues on systems with different default encodings, such as Windows.

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