refactor(scan): deduplicate insecure_full.json loading

[Rafael-Jost]

Description

safety/scan/ecosystems/python/main.py loaded the insecure_full.json
cache in three places with duplicated code: twice inside
PythonFile.__find_dependency_vulnerabilities__ and once in
PythonFile.remediate. The cache name, the skip_time_verification=True
argument and the DEBUG log message were repeated at each site, so any
change to how that cache is read had to be made in three places.

This extracts the lookup into a PythonFile._load_full_db() helper.

Two notes on the approach:

• The early return stays at each call site rather than moving into the
  helper. Returning from inside the helper would let callers continue
  executing with db_full unset.
• remediate() previously loaded the same file without logging. It now
  emits the same DEBUG message as the other two call sites when the cache
  is unavailable.

No other behavior change.

Type of Change

• Bug fix
• New feature
• Documentation update
• Refactor
• Other (please describe):

Related Issues

Part of #575

Testing

• Tests added or updated
• No tests required

Adds TestLoadFullDb to tests/scan/ecosystems/python/test_main.py,
covering both outcomes:

• cache hit — the loaded database is returned as-is, and get_from_cache
  is called with db_name="insecure_full.json", skip_time_verification=True
• cache miss — None is returned and the DEBUG message is logged

pytest tests/scan/ecosystems/python/test_main.py passes locally (7 tests,
Python 3.14.3).

Checklist

• Code is well-documented
• Changelog is updated (if needed)
• No sensitive information (e.g., keys, credentials) is included in the code
• All PR feedback is addressed

Additional Notes

CHANGELOG.md was intentionally left untouched — it states it is generated
automatically and should not be edited manually.

ruff check reports pre-existing findings on both touched files
(UP006, UP035, B006, DTZ003, SIM102, SIM118, I001). They are
identical on main and this PR introduces no new ones, so I left them
alone to keep the diff focused. Happy to fix them in a separate PR if you'd
like. ruff format --check is clean on both files.

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