Contextual searches (OpenSearch) use catch-all circuit to fetch metadata

Steps to reproduce after esr 140:

1. Switch browser.tordomainisolator.loglevel to All.
2. Visit https://developer.mozilla.org.
3. Clear the urlbar to show the unified search button.
4. Open the search menu and select "MDN Web Docs".

Alternatively:

4. Start a search term and select the "Search with MDN Web Docs" item.

Alternatively, in Tor Browser 14.5:

4. Start a search term and select the "Add search engine "MDN Web Docs"" item.

In the browser console, you will get a message that the catch-all circuit (--unknown--:0) was used to fetch:

1. https://developer.mozilla.org/opensearch.xml.
2. http://o.pki.goog/s/wr3/PDs.
3. https://developer.mozilla.org/favicon.ico.

I feel like these should all either user:

1. A new circuit.
2. Or, the circuit of the current web page.

NOTE: Whilst this existed in 14.5, esr 140 adds a more prominent path, which is meant to be a non-permanent one-off use of a search engine.

/cc @pierov @ma1 @morgan