Add StreamServer authentication test coverage to grpc_authn_interceptor_test.go

[coderabbitai]

Summary

The GrpcAuthnInterceptor.StreamServer entrypoint is security-critical but currently has no unit-test coverage. A regression there could silently allow unauthenticated access on all streaming RPCs, bypassing the JWT authentication hardening introduced in PR osac-project/fulfillment-service#685.

Severity: High — unauthenticated streaming calls are possible if StreamServer has a logic gap.

Work required

Add StreamServer test cases to internal/auth/grpc_authn_interceptor_test.go that mirror the existing unary cases:

• Rejects missing Authorization header on a private (non-anonymous) method → Unauthenticated
• Rejects unsupported authorization scheme → Unauthenticated
• Returns JWT validator error as a gRPC Unauthenticated status, handler not invoked
• Rejects multiple Authorization headers → Unauthenticated
• Allows anonymous methods without an Authorization header
• Rejects invalid token on an anonymous method → Unauthenticated
• Stores validated token in the stream context on success (TokenFromContext)

Use NewGrpcAuthnInterceptor(), MakeTokenString/MakeTokenObject/TokenFromContext, and MockJwtValidator with a small fake grpc.ServerStream that injects metadata via metadata.NewIncomingContext.

References

• PR: NO-ISSUE: Replace Authorino with built-in JWT authentication and OPA authorization fulfillment-service#685
• Review comment: NO-ISSUE: Replace Authorino with built-in JWT authentication and OPA authorization fulfillment-service#685 (comment)
• Requested by: @jhernand