Raise error on PSBTv0 missing PSBT_GLOBAL_UNSIGNED_TX

[KY-U]

PSBTv0 requires the field PSBT_GLOBAL_UNSIGNED_TX (0x00). However, the current parsing logic does not outright reject a PSBTv0 omiting that field. The resulting object's tx property then returns an empty default transaction (version=2, locktime=0, inputs=0 and outputs=0).

This PR raises an error when a PSBTv0 is missing the required PSBT_GLOBAL_UNSIGNED_TX field.

[Copilot]

Pull request overview

This pull request tightens PSBT parsing to reject PSBTv0 payloads that omit the required PSBT_GLOBAL_UNSIGNED_TX (0x00) field, preventing callers from accidentally working with a synthesized empty default transaction.

Changes:

• Add a validation in PSBT.read_from() to raise PSBTError when parsing a PSBTv0 that has no global unsigned transaction.
• Add a regression test vector for a minimal PSBTv0 missing the unsigned transaction field.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 1 comment.

File	Description
src/embit/psbt.py	Adds a guard to error out when a PSBTv0 is missing PSBT_GLOBAL_UNSIGNED_TX.
tests/tests/test_psbt.py	Adds an invalid vector exercising the missing-unsigned-tx case.

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[KY-U]

I've included both the implicit and explicit v0 on the version check: version in (None, 0). However, I'm not sure whether treating every non-v2 version on the v0 validation is conceptually correct. I think it makes sense that an explicit v0 should follow the rule, while other values should probably be rejected as unsupported rather than fall on the same v0 rule.

I could add an extra case to explicitly fail on non-supported versions:

if version not in (None, 0, 2):
    raise PSBTError("Unsupported PSBT version")

[Copilot]

Pull request overview

Copilot reviewed 2 out of 2 changed files in this pull request and generated no new comments.