Taproot descriptor parsing fails on uppercase hex characters

[erickcestari]

After differential fuzzing using bitcoinfuzz with NBitcoin, rust-miniscript and Bitcoin Core, I discovered that NBitcoin fails to parse valid Taproot descriptors containing uppercase hex characters in the internal key, breaking compatibility with Bitcoin Core and rust-miniscript.

Reproduction

This valid Taproot descriptor fails:

tr(01b2b2472Dbbbbbbbbbcbbbbbcbbbbbbcbbbbbbbbbbbbbcbbbbbbbbbbbbcbbbb)

Root Cause

The hex parser in Parse.cs line 8 only accepts lowercase hex:

private const string ValidHex = "0123456789abcdef";  // Missing A-F

When parsing the Taproot internal key (x-only pubkey), PPubkeyXOnly() uses Parse.Hex.Repeat(64) which rejects uppercase hex characters.