descriptors: check duplicate keys in all multipath Miniscript branches #35742

Fixes #35629.

Multipath Miniscript descriptors performed duplicate-key sanity checking while parsing the template, when key comparison used the first multipath branch. As a result, duplicate keys present only in a later branch were accepted.

Select each expanded multipath branch and rerun the existing duplicate-key check before constructing the final descriptors. If a branch contains a duplicate, report the offending concrete Miniscript expression.

The regression test covers:

• a valid two-branch wsh control;
• a duplicate in the second wsh branch;
• a duplicate in the third of three tr branches.

Tested with:

build/bin/test_bitcoin --run_test=descriptor_tests/multipath_miniscript_duplicate_keys
build/bin/test_bitcoin --run_test=descriptor_tests
build/bin/test_bitcoin

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Code Coverage & Benchmarks

For details see: https://corecheck.dev/bitcoin/bitcoin/pulls/35742.

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Conflicts

Reviewers, this pull request conflicts with the following ones:

• #34697 (descriptor: fix musig() duplicate key checks and doubled PSBT origin paths by shuv-amp)

If you consider this pull request important, please also help to review the conflicting pull requests. Ideally, start with the one that should be merged first.

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