Fix pull-through distribution base paths

[gerrod3]

What this does

• Restores pulpcore base-path overlap validation for container distributions.
• Stores UUID base paths for new pull-through distributions and marks them with a reserved pulp label.
• Looks up marked pull-through distributions by name, then falls back to the legacy base-path lookup.
• Adds a repair command for existing pull-through distributions and reports entries that need manual repair or still overlap another distribution.
• Adds Django warning pulp_container.W001 until operators run the repair command.

Repair command

pulpcore-manager container-repair-pull-through-distributions

The command repairs distributions where name matches base_path. It leaves mismatched distributions untouched and reports them for manual handling.

Tests

• Full unit suite: 43 passed, 3 subtests passed.
• API persistence test for the UUID base path and marker label passed.
• Ruff and formatting checks passed.
• Django system checks passed.

Closes #2494

[mdellweg]

Thank you for looking into this!