feat(external-call): Add transaction encoding/decoding for external calls

[trusch]

Summary

This PR adds serialization support for external call results in transactions. It enables transactions with external calls to be persisted, transmitted between participants, and reconstructed.

Requires #439

Changes

Transaction Coder (TransactionCoder.scala)

Encoding

private def encodeExternalCallResult(result: ExternalCallResult): Proto.ExternalCallResult =
  Proto.ExternalCallResult.newBuilder()
    .setExtensionId(result.extensionId)
    .setFunctionId(result.functionId)
    .setConfig(ByteString.copyFrom(result.config.toByteArray))
    .setInput(ByteString.copyFrom(result.input.toByteArray))
    .setOutput(ByteString.copyFrom(result.output.toByteArray))
    .build()

private def encodeExerciseNode(node: Node.Exercise, ...): Proto.Node = {
  val builder = Proto.Node.Exercise.newBuilder()
  // ... existing fields ...
  node.externalCallResults.foreach { result =>
    builder.addExternalCallResults(encodeExternalCallResult(result))
  }
  // ...
}

Decoding

private def decodeExternalCallResult(proto: Proto.ExternalCallResult): ExternalCallResult =
  ExternalCallResult(
    extensionId = proto.getExtensionId,
    functionId = proto.getFunctionId,
    config = Bytes.fromByteString(proto.getConfig),
    input = Bytes.fromByteString(proto.getInput),
    output = Bytes.fromByteString(proto.getOutput)
  )

private def decodeExerciseNode(proto: Proto.Node.Exercise, ...): Node.Exercise = {
  // ... existing fields ...
  val externalCallResults = proto.getExternalCallResultsList.asScala.map(decodeExternalCallResult).toSeq
  Node.Exercise(..., externalCallResults = externalCallResults)
}

Backward Compatibility

• Transactions without external calls decode normally (empty list)
• Proto field is repeated so missing field = empty list
• No migration needed for existing transactions

Testing

• TransactionCoderSpec.scala - Round-trip encoding tests
• ContractStateMachineSpec.scala - State machine with external calls
• ValidationSpec.scala - Validation of encoded transactions
• TestNodeBuilder.scala - Test helpers for building nodes

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