Musig2 fields followups #32412

1. 
   rkrux commented at 8:59 am on May 3, 2025: contributor
2. psbt: improve musig2 partial sig & pubnonce key size check formatting

   Follow up from MuSig2 Fields PR - #31247
   Ref: https://github.com/bitcoin/bitcoin/pull/31247#discussion_r2050497355

   7ca186c510
3. psbt: update invalid psbt error messages in functional test vectors

   The error messages related to Musig2 pubnonce & partial sig key sizes
   are updated to match the full error message so that there is more
   stringent error checking.
   
   The test passed earlier because `assert_raises_rpc_error` function in
   the functional tests checks for susbstring of the error messages as well.
   
   Follow up from previous PR #31247.
   Ref: https://github.com/bitcoin/bitcoin/pull/31247#discussion_r2050692514

   10fd2aeb62
4. 
   DrahtBot commented at 8:59 am on May 3, 2025: contributor

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

   Code Coverage & Benchmarks

   For details see: https://corecheck.dev/bitcoin/bitcoin/pulls/32412.

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