feat(external-call): Add Canton protocol support for external calls

[trusch]

Summary

This PR adds Canton-specific protocol support for external calls, including ActionDescription extensions, view participant data, and transaction hashing.

Requires #440

Changes

Proto Definition (participant_transaction.proto)

message ActionDescription {
  message ExerciseActionDescription {
    // ... existing fields ...
    repeated ExternalCallResult external_call_results = 12;
  }
}

ActionDescription (ActionDescription.scala)

Extended ExerciseActionDescription to include external call results:

final case class ExerciseActionDescription(
    inputContractId: LfContractId,
    templateId: LfTemplateId,
    choice: LfChoiceName,
    interfaceId: Option[LfInterfaceId],
    packagePreference: Set[LfPackageId],
    chosenValue: LfVersioned[Value],
    actors: Set[LfPartyId],
    byKey: Boolean,
    seed: LfHash,
    failed: Boolean,
    externalCallResults: Seq[ExternalCallResult]  // NEW
) extends ActionDescription

ViewParticipantData (ViewParticipantData.scala)

• Updated to propagate external call results through view construction

NodeHashBuilder (NodeHashBuilder.scala)

Added external call results to exercise node hash:

private def addExerciseNode(node: Node.Exercise): this.type = {
  // ... existing fields ...
  withContext("External Call Results") {
    _.addIterator(node.externalCallResults.iterator, node.externalCallResults.size) {
      (builder, result) =>
        builder
          .addString(result.extensionId)
          .addString(result.functionId)
          .addBytes(result.config)
          .addBytes(result.input)
          .addBytes(result.output)
    }
  }
}

View Decomposition

External call results are aggregated at the view level:

1. Each Exercise node in a view may have external calls
2. Results from all Exercise nodes in the view core are collected
3. ActionDescription stores the aggregated results
4. ModelConformanceChecker validates during reinterpretation

Hashing

External call results are included in the transaction hash to ensure:

• All participants compute the same hash
• Tampering with results is detectable
• Consensus requires matching results

Testing

• ExampleTransactionFactory.scala - Factory for test transactions
• WellFormedTransactionTest.scala - Well-formedness checks
• NodeHashV1Test.scala - Hash computation tests

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