Removed unnecessary check for tapInternalKey for signature validations of taproot inputs

[summraznboi]

getTaprootHashesForSig is an overly reused function for both signing as well as signature validation.
To better make each path clearer, we created two wrappers on top to clearly state its intent, getTaprootHashesForSigValidation and getTaprootHashesForSigning. Note that getAllTaprootHashesForSig is just renamed to getAllTaprootHashesForSigValidation, since there isn't a corresponding usage for signing.

This separation allows for us to clearly see that for sig validation, we only need to see the existence of tapKeySig to know that this is a key spend path. This is especially important for nonstandard key spend path patterns such as MuSig2 or other offchain multisig signing protocols.

[Copilot]

Pull Request Overview

This PR refactors the taproot signature hash generation logic to separate signing and validation concerns. Previously, a single function getTaprootHashesForSig was used for both purposes, making the code path unclear.

• Split getTaprootHashesForSig into two wrapper functions: getTaprootHashesForSigning and getTaprootHashesForSigValidation
• Updated key spend detection logic to use tapKeySig for validation and tapInternalKey for signing
• Renamed getAllTaprootHashesForSig to getAllTaprootHashesForSigValidation for clarity

Reviewed Changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 2 comments.

File	Description
ts_src/psbt.ts	Main TypeScript implementation with function splitting and key spend logic updates
src/esm/psbt.js	ES module version with identical refactoring changes
src/cjs/psbt.cjs	CommonJS version with identical refactoring changes

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[summraznboi]

Thanks for merging this! It would be great to have a bug fix release soon so we can make use of it :)