ci: establish a hermetic unit-test suite

[linhongyu510]

Summary

This is a focused first phase of #819:

• establish an allowlisted tests/unit suite that does not require provider SDK extras, credentials, databases, downloads, or network sockets
• split the dataset tests so download and large-dataset iteration cases live under tests/e2e
• add unit-test and e2e-test Make targets while keeping unittest as a compatibility alias
• run the hermetic unit suite in pull-request CI
• document the temporary boundary between classified unit/E2E tests and remaining legacy provider tests

Scope

This PR intentionally does not claim to complete the provider-by-provider reorganization proposed in #819. Tests still directly under tests/ remain legacy and unclassified; they can be split in follow-up changes.

Validation

Validated in fresh isolated environments with only the project and .[test] installed:

• Python 3.11: 255 passed with sockets disabled
• Python 3.12: 255 passed with sockets disabled
• Black: 225 source files unchanged
• Ruff: all checks passed
• git diff --check: passed

Progresses #819.

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[linhongyu510]

/assign @XuanYang-cn

[linhongyu510]

Rebased this PR onto the latest main and resolved the dataset-test split conflict. The branch is now conflict-free (mergeable).

The conflict came from new dataset tests added upstream after this PR was opened. I preserved all 15 upstream dataset tests and kept the intended hermetic boundary:

• 12 local-only tests now live in tests/unit/test_dataset.py
• 3 download / large-dataset iteration tests remain in tests/e2e/test_dataset.py

I also updated two existing FTS unit-test stubs with the newly required config.case_config.k field introduced upstream. No production logic was changed.

Local verification on Python 3.11:

255 passed in 3.08s

Black and import-order checks also pass for the touched test files. Could a maintainer take another look when convenient? Thanks!