fix: handle compound extensions in LinkExtractor

[axelray-dev]

Proposed change

_url_extension() returns only the last suffix of a URL path segment (e.g. "gz" for dataset.tar.gz). This means compound extensions like "tar.gz" that are already listed in IGNORED_EXTENSIONS never match during deny_extensions filtering, so archive files such as .tar.gz and .tar.bz2 pass through the extractor undetected.

This PR adds a _url_extensions() helper that returns all dot-suffixes for the last path segment (e.g. {"gz", "tar.gz"} for dataset.tar.gz) and uses it in _url_passes() so compound extensions are correctly rejected. The existing _url_extension() function is preserved unchanged for backward compatibility.

Type of change:

• Bugfix (non-breaking change which fixes an issue)
• Dependency upgrade
• New integration (thank you!)
• New feature (which adds functionality to an existing integration)
• Deprecation (breaking change to happen in the future)
• Breaking change (fix/feature causing existing functionality to break)
• Code quality improvements to existing code or addition of tests
• Add or change doctests?
• Documentation change?

Additional information

• This PR fixes or closes an issue: fixes [BUG] [Good First Issue] LinkExtractor never filters .tar.gz links #349
• Tests run: pytest tests/spiders/test_links.py -v -- 34/34 passed
• Lint: ruff check and ruff format --check both clean
• Whitespace: git diff --check clean

Checklist:

• I have read CONTRIBUTING.md.
• This pull request is all my own work -- I have not plagiarized.
• I know that pull requests will not be merged if they fail the automated tests.
• All new Python files are placed inside an existing directory.
• All filenames are in all lowercase characters with no spaces or dashes.
• All functions and variable names follow Python naming conventions.
• All function parameters and return values are annotated with Python type hints.
• All functions have doc-strings.

[yetval]

Confirmed this fixes it. .tar.gz gets dropped by default now, and explicit deny_extensions={"tar.gz"} works too while plain .gz is still kept. Tests cover the cases well.

Only nit: _url_extension (singular) has no callers after this, so it could be dropped, but no big deal. Thanks for the fix!