Refactor JtdToEsmCodegenTest to use GraalVM Polyglot instead of bun

- Remove all bun-specific code (ProcessBuilder runners, isBunAvailable checks)
- Replace with GraalVM Polyglot JS in-process execution via jsContext()/evalModule()
- Add GraalVM helper methods: jsContext(), evalModule(), errCount()
- Simplify test schemas (inline them instead of separate variables)
- Add generatedDiscriminatorValidatorWorks test
- Add JTD_CODEGEN_SPEC.md documentation
- All 17 tests pass, no external JS runtime required

Author: simbo1905