feat(parsers): add fix_version support to Generic Findings Import

[vvpoglazov]

Description:
The fix_version field exists on the Finding model (CharField, max_length=100)
and is already handled by the reimporter (default_reimporter.py lines 683, 803),
but was missing from the Generic Findings Import parser for both JSON and CSV formats.

This meant users could see and edit fix_version in the UI, but could not
populate it via Generic Findings Import.

Changes:

• json_parser.py: Added fix_version to the allowed fields set
• csv_parser.py: Added fix_version field parsing
• Added test scan files (JSON + CSV) with fix_version data
• Added unit tests for both formats
• Updated parser documentation

Test results:
Added two unit tests in unittests/tools/test_generic_parser.py:

• test_parse_json_with_fix_version
• test_parse_csv_with_fix_version

Also manually verified via API reimport that fix_version populates correctly
on findings.

Checklist:

• Make sure to rebase your PR against the very latest dev.
• Features/Changes should be submitted against the dev.
• Bugfixes should be submitted against the bugfix branch.
• Give a meaningful name to your PR, as it may end up being used in the release notes.
• Your code is flake8 compliant.
• Your code is python 3.13 compliant.
• If this is a new feature and not a bug fix, you've included the proper documentation in the docs at https://github.com/DefectDojo/django-DefectDojo/tree/dev/docs as part of this PR.
• Model changes must include the necessary migrations in the dojo/db_migrations folder.
• Add applicable tests to the unit tests.
• Add the proper label to categorize your PR.

[mtesauro]

Approved