Defer transaction signing until user clicks Send

[151henry151]

Fixes #30070

When creating an unsigned PSBT from the GUI, the transaction was already signed during preparation, causing legacy inputs to have non-empty scriptSig fields. The PSBT parser then rejects them.

This defers signing until the user clicks "Send" instead of signing during preparation. Fee calculation still works since transactions can be created without signing.

Follows the approach suggested by @achow101 in the issue comments.

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Concept ACK	achow101

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Conflicts

Reviewers, this pull request conflicts with the following ones:

• #bitcoin/bitcoin/33112 (wallet: relax external_signer flag constraints by Sjors)

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[151henry151]

It looks like the one failing check is a CI issue with disk space too low. I'm not sure how to address that. I think the code of the PR is correct.

[151henry151]

@achow101 would you like to check this out and let me know if it looks OK?

[151henry151]

Should I push an empty commit to trigger a re-run of the CI here, or is there some other action needed?

[hebasto]

Should I push an empty commit to trigger a re-run of the CI here, or is there some other action needed?

No action needed.

[hebasto]

cc @achow101 @furszy

[achow101]

Concept ACK

[151henry151]

For the rebase, I kept master's new createTransaction usage and error flow, wired in the defer-signing logic (sign parameter), and re-added the AmountExceedsBalance check after a successful result. I initially missed updating that to AmountExceedsBalance, but I’ve corrected it. If anyone wants to check this out and let me know if it looks correct, I'd appreciate any feedback. Thanks