fix: marinade multisig dapp requests by preserving original message bytes

[taran-a]

Explanation

Fixes multisig dapp transaction signing by preserving the original transaction payload for external dapp origins.

• Decode dapp-origin signTransaction and signAndSendTransaction requests as transactions directly, whether the payload is a full serialized transaction or a bare compiled transaction message.
• Append only the snap account's signature, preserving existing signatures, compiled messageBytes, account ordering, and instruction account indices.
• Keep the existing MetaMask-origin signing flow unchanged.
• Add coverage for codec decoding, signer byte preservation, and wallet origin handling.

References

• Fixes WPN-1035

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

[taran-a]

@metamaskbot publish-preview

[github-actions]

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@metamask-previews/bitcoin-wallet-snap@2.0.1-preview-c5b54f00
@metamask-previews/snap-networks-utils@1.0.0-preview-c5b54f00
@metamask-previews/solana-wallet-snap@6.0.0-preview-c5b54f00
@metamask-previews/tron-wallet-snap@3.1.0-preview-c5b54f00