Merge #18224: Make AnalyzePSBT next role calculation simple, correct

1ef28b4f7cfba410fef524def1dac24bbc4086ca Make AnalyzePSBT next role calculation simple, correct (Gregory Sanders)

Pull request description:

  Sniped test and alternative to https://github.com/bitcoin/bitcoin/pull/18220

  Sjors documenting the issue:
  ```
  A PSBT signed by ColdCard was analyzed as follows (see #17509 (comment))

  {
    "inputs": [
      {
        "has_utxo": true,
        "is_final": false,
        "next": "finalizer"
      }
    ],
    "estimated_vsize": 141,
    "estimated_feerate": 1e-05,
    "fee": 1.41e-06,
    "next": "signer"
  }
  I changed AnalyzePSBT so that it returns "next": "finalizer" instead.
  ```

  It makes it much clearer that the role has been decided before hitting the `calc_fee` block, and groups all state-deciding in one spot instead of 2.

  Note that this assumes that PSBT roles are a complete ordering, which for now and in the future seems to be a correct assumption.

ACKs for top commit:
  Sjors:
    ACK 1ef28b4f7cfba410fef524def1dac24bbc4086ca, much nicer. Don't forget to document the bug fix.
  achow101:
    ACK 1ef28b4f7cfba410fef524def1dac24bbc4086ca
  Empact:
    ACK https://github.com/bitcoin/bitcoin/pull/18224/commits/1ef28b4f7cfba410fef524def1dac24bbc4086ca

Tree-SHA512: 22ba4234985c6f9c1445b14565c71268cfaa121c4ef000ee3d5117212b09442dee8d46d9701bceddaf355263fe25dfe40def2ef614d4f2fe66c9ce876cb49934

0 files changed, 0 insertions, 0 deletions