Align YoutubeIngestor#discover_feed_url visibility with superclass

[Copilot]

Summary of changes

The linked review comment requested addressing only #discussion_r3287496889: discover_feed_url in YoutubeIngestor should not be part of the public ingestor API. This PR applies that scope exactly by making the method private to match MaterialRSSIngestor.

• Visibility consistency
  • Added private in Ingestors::YoutubeIngestor before discover_feed_url.
  • No behavioral or logic changes beyond method visibility.

class YoutubeIngestor < MaterialRSSIngestor
  # ...

  private

  def discover_feed_url(content, base_url)
    # ...
  end
end

Motivation and context

Review feedback on PR #1276 identified a visibility mismatch between subclass and superclass for the same hook method; this change resolves that mismatch with minimal surface-area impact.

Screenshots

N/A (no UI change)

Checklist

• I have read and followed the CONTRIBUTING guide.
• I confirm that I have the authority necessary to make this contribution on behalf of its copyright owner and agree to license it to the TeSS codebase under the BSD license.