psbt: update PSBTv2 modifiable flags after signing #36479

Signing a PSBTv2 currently leaves its transaction modifiable flags unchanged. This can tell other software that inputs or outputs can still be changed even when those changes would invalidate the signature. The flag indicating a SIGHASH_SINGLE signature can also be missing. This change updates the flags when a signature is added, according to its sighash type and BIP370. It handles partial signatures too. Calls that only exchange MuSig2 nonces or add no signature leave the flags unchanged. The regression tests cover different sighash types, P2WPKH, Taproot, multisig, and MuSig2 signing, along with PSBTv0 and missing-field controls.

Testing:

• Full rpc_psbt.py passed.
• CTest: 377 passed, 1 skipped because its external test data was unavailable.

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External sites

• Code coverage & benchmarks: https://corecheck.dev/bitcoin/bitcoin/pulls/36479
• Ralph review <img src=https://review.fish.foo/assets/bitcoin/bitcoin/36479.png alt="Ralph review status"> https://git.fish.foo/bitcoin/bitcoin/pulls/36479#author-ralph

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Reviews

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To preserve the time of reviewers, this project requires authors to understand the code they are submitting. Given that the description seems to be mostly LLM generated, I don't think that is the case, so I am closing this again.