Function negotiate_header_extensions 

fn negotiate_header_extensions(
    consumable_parameters: &MediaStream,
    consumer_capabilities: &MediaCapabilities,
) -> Vec<HeaderExtension>

This is a URI-level capability intersection only.

It does NOT perform full SDP extmap negotiation:

• no direction filtering
• no id collision checks
• no BUNDLE-wide extmap consistency checks

Those rules belong to the SDP/signaling edge. This helper only answers whether the typed RTP model should keep an extension URI at all.