[Compiler][gRPC] Test plan and cross-language smoke tests #3277
Description
Parent issue: #3266
Scope
Add unit and integration tests for gRPC service support.
Detailed tasks
- Parser tests for FDL/proto/fbs service syntax.
- IR validation tests for name/type/streaming rules.
- Golden codegen tests for service output files and key signatures.
- Optional cross-language smoke tests using Fory codec over gRPC.
Notes
Keep tests focused and fast; avoid unnecessary integration unless required.