fix: return 404 for missing versioning parents

[Dexter2099]

Changes

• Return 404 when feature-versioning permission checks encounter a missing nested environment or feature parent.
• Add focused regression coverage for missing parent resources in the feature-versioning permission classes.

Root cause

DRF runs permission checks before the feature-versioning view initial() methods. These permission classes used direct Environment.objects.get(...) / Feature.objects.get(...) lookups, so missing nested parents could raise raw DoesNotExist exceptions instead of returning a 404 response.

Fixes #6556

Testing

• uv run ruff check features/versioning/permissions.py tests/unit/features/versioning/test_unit_versioning_permissions.py
• PYTEST_DISABLE_PLUGIN_AUTOLOAD=1 DATABASE_URL=postgresql://postgres:password@localhost:5433/flagsmith ANALYTICS_DATABASE_URL=postgresql://postgres:password@localhost:5433/analytics uv run pytest -p django -p xdist.plugin -p pytest_mock tests/unit/features/versioning/test_unit_versioning_permissions.py -n0

Note: I used the explicit pytest plugins above because local Windows plugin autoload hits a pyreadline compatibility issue. The broader versioning view module also hits a Windows-only gunicorn/fcntl import path in this local environment.

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