Add compilation test to verify generated code compiles successfully

[halotukozak]

Description

Current generator tests verify output via string/snapshot comparison but never compile the generated code. A compilation test would catch issues like missing imports, invalid syntax, or type mismatches that snapshot tests miss. Use Kotlin Compile Testing (e.g. com.github.tschuchortdev:kotlin-compile-testing) or a similar approach to compile generated output against Ktor/kotlinx-serialization dependencies.

Acceptance criteria

• At least one test generates code from a representative OpenAPI spec and compiles it
• Compilation includes necessary runtime dependencies (Ktor client, kotlinx-serialization, Arrow)
• Test fails if generated code has syntax errors, missing imports, or type mismatches
• Runs as part of the standard ./gradlew test suite