Fix env var default value syntax (${VAR || "default"})

[omnarayan]

Summary

• Fix ${VAR || "default"} and ${VAR ?? "fallback"} env var patterns that Maestro supports but maestro-runner wasn't resolving
• Undefined JS variables now auto-define as undefined on ReferenceError (retry loop), matching Maestro's GraalJS Proxy behavior
• Flow config env: values and step-level env: values are now expanded before being set

Test plan

• Unit tests: 8 new engine tests (||, ??, ternary, chain, mixed), 1 new scripting test for withEnvVars expansion
• All existing tests pass
• E2E: test flow with appId: ${APP_ID || "com.example.app"} and env: { USER_ID: ${USER_ID || "test_user_1"} }

Fixes #49, fixes #50