fix: show filled PSBT in signPsbt confirmation

[jeremytsng]

Explanation

The signPsbt confirmation showed the unfilled template PSBT supplied by the dApp, while a freshly filled PSBT (with the dApp's feeRate) was the one actually signed. The user could not see the inputs, real fee, or feeRate before approving, allowing a malicious dApp to spoof the displayed transaction.

Fill the PSBT before showing the confirmation when options.fill is true, and pass fill=false to the inner signPsbt so the use case does not double-fill (which would re-introduce divergence between displayed and signed transaction).

Screenshots

References

Checklist

• I've updated the test suite for new or updated code as appropriate
• I've updated documentation (JSDoc, Markdown, etc.) for new or updated code as appropriate
• I've communicated my changes to consumers by updating changelogs for packages I've changed
• I've introduced breaking changes in this PR and have prepared draft pull requests for clients and consumer packages to resolve them

---

Note

Medium Risk
Touches PSBT signing flow and confirmation data, so a mistake could cause users to approve/sign a different transaction than displayed. Changes are localized and covered by new unit tests for fill ordering and failure behavior.

Overview
Fixes KeyringRequestHandler#signPsbt so when options.fill is true it fills the PSBT before displaying the confirmation, ensuring the user reviews the same transaction that will be signed.

The handler now signs the filled PSBT while forcing { fill: false } for the inner signPsbt call to avoid double-filling and reintroducing divergence; tests were updated/added to assert call order, parsing behavior, and that failures in fillPsbt abort confirmation and signing. Changelog updated to note the fix.

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[jeremytsng]

@metamaskbot publish-preview

[github-actions]

Preview builds have been published. See these instructions for more information about preview builds.

Expand for full list of packages and versions.

{
  "@metamask-previews/bitcoin-wallet-snap": "1.10.1-preview-bd78f2a"
}

[Battambang]

Please, consider:

• Create the preview-build in the PR and testing it with a bump in the MM client.
• Add in the PR the screenshot of the dialog box.
• Update the changelod.MD about this user facing change.
• Have QA acceptance feedback for merge to main.