Add NAME_FILTER_OPTIONS to FileContent relative_path filter

[dkliban]

Problem

FileContentFilter in pulp_file/app/viewsets.py only supports exact matching on the relative_path field:

class Meta:
    model = FileContent
    fields = ["relative_path", "sha256"]

This means API consumers cannot filter file content by path patterns — there is no support for contains, startswith, regex, or other lookup expressions on relative_path.

Other content filters in pulpcore already use NAME_FILTER_OPTIONS for name/path fields (e.g., ContentFilter), so FileContentFilter is inconsistent with the rest of the API.

Proposed fix

Use NAME_FILTER_OPTIONS for relative_path to enable the full set of lookup expressions:

from pulpcore.app.viewsets.base import NAME_FILTER_OPTIONS

class Meta:
    model = FileContent
    fields = {
        "relative_path": NAME_FILTER_OPTIONS,
        "digest": ["exact"],
    }

This enables: exact, iexact, in, contains, icontains, startswith, istartswith, regex, iregex.

The existing sha256 = CharFilter(field_name="digest") alias on the filter class continues to provide backwards-compatible ?sha256= query parameter support.

Use case

Deployments managing large file repositories need to search for content by path prefix (e.g., all files under packages/python/) or by pattern (e.g., all .rpm files). Currently this requires fetching all file content and filtering client-side.