fix: include BDK cause in PSBT build error message

[cloudonshore]

Problem

The "Failed to build PSBT from template" error is one of the top errors in Unified SwapBridge (~7.5% of failures). The root cause from BDK (e.g. InsufficientFunds, OutputBelowDustLimit, NoUtxosSelected) is lost when crossing the snap JSON-RPC boundary because:

1. HandlerMiddleware re-throws as InvalidParamsError which doesn't accept a cause parameter (line 71)
2. emitTrackingError checks error.cause instanceof Error but BDK WASM errors are CodifiedError plain objects, not Error instances (line 308)

This makes it impossible to diagnose why the PSBT rebuild fails in Sentry or Mixpanel.

Solution

Append the BDK cause message to the ValidationError message string so it survives serialization through the RPC boundary and appears in Sentry/Mixpanel. Handles both Error instances and BDK's CodifiedError plain objects.

After this change, errors will appear as:

Validation failed: Failed to build PSBT from template: InsufficientFunds

instead of:

Validation failed: Failed to build PSBT from template

Test plan

• Existing tests updated and passing
• New test for non-Error (CodifiedError) cause extraction
• Verify in Sentry that new errors include the BDK cause message after deployment

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Note

Low Risk
Low risk: changes are limited to error-message composition and accompanying unit tests, with no effect on PSBT construction logic. Main potential impact is on any consumers asserting exact error strings.

Overview
PSBT build failures now surface the underlying cause message. When AccountUseCases.#fillPsbt fails to finish() a transaction, the thrown ValidationError message is updated from a generic "Failed to build PSBT from template" to include the caught error’s .message (or unknown cause when absent), ensuring the reason survives RPC serialization.

Tests are updated/added to cover Error causes, non-Error object causes, and missing-message causes, and the change is documented in the Unreleased changelog (with the snap manifest shasum updated for the new bundle).

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