golem-scala-code-generation

Add the method in the shared codegen module under sdks/scala/codegen, treating RpcCodegen.scala, AgentClientRuntime.scala, and InvocationMetadata.scala as authoritative for signatures. Then run the codegen tests on both Scala versions and compile a consumer project.

Keep all semantic generation in the shared codegen module and make the sbt and Mill plugins thin wrappers that only handle source collection, logging, output directories, formatting, and task wiring. Check existing task names and output handling before adding hooks.

The shared codegen sources are consumed by sbt 1.x on Scala 2.12.21 and by the repository build and Mill plugin on Scala 3.8.2, so avoid Scala-3-only syntax. Mill consumer fixtures compile generated application code with Scala 3.3.7, which is a fixture setting, not the codegen host version.

Durable clients expose get and phantom constructors, while ephemeral clients expose newPhantom. Durable apply returns Future[A] and ephemeral apply returns Future[InvocationResult[A]]; cancelable and trigger/schedule operations also return different receipt types per mode.

Run the affected generator tests with sbt ++3.8.2; codegen/test, cross-test with ++2.12.21! for the sbt plugin, and compile a consumer with testAgents/fastLinkJS. For Mill changes, use the tasks in sdks/scala/mill/README.md and the fixtures in mill/build.mill.