Guard provider-dependent tests in airflow-core for standalone core testing

[ShubhamGondane]

Adds pytest.importorskip guards to core tests that import optional provider packages so they skip gracefully when providers are not installed. Removes 6 provider packages from airflow-core dev dependencies. This enables cd airflow-core && uv sync && pytest to work standalone. The pytest.importorskip pattern is already established in the codebase (test_executor_loader.py, test_connection.py). These tests continue to run in CI and in full workspace environments (uv sync --all-packages).

The remaining provider-dependent tests all test core functionality (executor loading, serialization, CLI parsing) using provider classes as test inputs. Previous PRs (#48113, #49178, #54635) already moved truly provider-specific tests. These belong in core and are guarded rather than moved.

closes: #60770

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Was generative AI tooling used to co-author this PR?

• Yes - Claude Code (claude-opus-4-6)

Generated-by: Claude Code (claude-opus-4-6) following the guidelines

[potiuk]

@ShubhamGondane This draft PR has had no activity for 5 weeks. Closing to keep the queue clean.

You are welcome to reopen and continue when you're ready. If you'd like to pick it back up, please rebase onto the current main branch first.

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