Extract ResourceEntityResolver HTTPS schema resolution fallback

[simonbasle]

This commit extracts the DTD/XSD remote lookup fallback from the
resolveEntity method into a protected method.

A warn logging statement is added to the extracted fallback in order to
make it clear that remote lookup happened.

Overriding the protected method would allow users to avoid this fallback
entirely if it isn't desirable, without the need to duplicate the local
resolution code.

Fixes gh-29662.

[simonbasle]

draft because I'm wondering about the extracted method's javadoc: it should probably list returning nulland throwing as override possibilities, but I'm not sure how to explain what each of these options does in a nutshell. would appreciate suggestions here

[scantor]

draft because I'm wondering about the extracted method's javadoc: it should probably list returning nulland throwing as override possibilities, but I'm not sure how to explain what each of these options does in a nutshell. would appreciate suggestions here

Refer to the EntityResolver's resolveEntity semantics as a reference perhaps? That's ultimately what you're triggering.

[sbrannen]

draft because I'm wondering about the extracted method's javadoc: it should probably list returning nulland throwing as override possibilities, but I'm not sure how to explain what each of these options does in a nutshell. would appreciate suggestions here

Refer to the EntityResolver's resolveEntity semantics as a reference perhaps? That's ultimately what you're triggering.

Returning null will trigger the default behavior of the parser.

Throwing an exception should cause the parser to fail (and skip any attempt to load the remote resource).

We should introduce tests for both use cases.

[sbrannen]

This has been merged into main in 5965917 and revised in 8df4f92.

Thanks, and... congratulations on having your first PR merged into the code base! 👍