descriptor: warn about multipath key reuse #36004

This PR adds a warning when a multipath descriptor causes a non-multipath key to be cloned across multiple branches.

For example, a descriptor such as:

wsh(multi(2,A/<0;1>/,B/0/))

currently reuses the same B/0/* key material across the generated branches.

The existing behavior is kept: these descriptors remain valid and are not rejected. The change only exposes a warning through getdescriptorinfo.

The warning is only triggered when a key provider is actually cloned to match a multipath expression. Cloning subdescriptors in tr() does not trigger the warning.

Tests:

• Added descriptor unit tests covering multipath key cloning.
• Added coverage for tr() internal-key vs subdescriptor cloning.
• Added a functional test for the getdescriptorinfo warning.
• Ran the descriptor unit tests and rpc_getdescriptorinfo.py successfully.

Fixes #35985

<!--e57a25ab6845829454e8d69fc972939a-->

The following sections might be updated with supplementary metadata relevant to reviewers and maintainers.

<!--006a51241073e994b41acfe9ec718e94-->

Code Coverage & Benchmarks

For details see: https://corecheck.dev/bitcoin/bitcoin/pulls/36004.

<!--021abf342d371248e50ceaed478a90ca-->

Reviews

See the guideline and AI policy for information on the review process. A summary of reviews will appear here.

<!--5faf32d7da4f0f540f40219e4f7537a3-->

Dupe of #36005, please instead review that one