Add Scala 3 code generation support (#2188)

[Tanveer17]

Summary

• Add ScalaVersion ADT to represent target Scala version (2.12, 2.13, 3)
• Add Scala3Compat utility for generating using clauses instead of implicit for Scala 3
• Add scalaVersion field to Context for version-aware code generation
• Update AkkaHttp generators to use Scala3Compat.implicitsClause
• Update Http4s client generator for Scala 3 compatibility

Closes #2188

Test plan

• Unit tests for ScalaVersion parsing and behavior (16 tests)
• Unit tests for Scala3Compat clause generation (13 tests)
• Unit tests verifying generated server code uses using for Scala 3 (5 tests)
• Unit tests verifying generated client code uses using for Scala 3 (4 tests)

Notes on Integration Testing

The current unit tests verify the AST structure contains correct using/implicit syntax based on Scala version. Full compilation-based integration testing (via
modules/sample-*) would require adding Scala 3 to the build infrastructure, as sample modules currently only compile with Scala 2.12/2.13.

Options for future work:

1. Add a single sample-scala3 module to validate compilation
2. Cross-compile existing samples with Scala 3

Happy to discuss the best approach for integration testing infrastructure.

Related PRs

• sbt-guardrail: Threading scalaVersion parameter (pending)
• module-pekko-http: Scala 3 support for Pekko HTTP generators (pending)

[blast-hardcheese]

phenomenal. Pardon my delay on review, I've been quite sick over the past week.

I've adjusted this to be a major bump to make the release simpler. We've been on RC for over a year while the module system burned in, which ended up also scope creeping in my head to also improving the ergonomics for the client codegen, but I think it'll actually be both easier and more sensible to just release a revised client API with a standard deprecation cycle (interfaces annotated with @deprecated, 2.x to drop the deprecated client API)