fix: map no trailers ok status to internal

[rumenov]

Motivation

The change updates gRPC status handling so that HTTP 200 responses missing grpc-status trailers are treated as internal errors instead of silently completing. A new integration test simulates a truncated response to ensure clients surface the issue. This prevents data loss scenarios from being masked as successful completions.
Clients will now see explicit internal errors when a streaming response ends without grpc-status trailers, preventing silent data loss. The new integration test guards this behavior against future regressions.

Solution

1. gRPC Status Mapping
   Adjusted tonic’s infer_grpc_status logic to treat missing trailers as protocol violations.
   http 200 responses without grpc-status trailers now return an internal Status error instead of appearing as clean end-of-stream.
   Expanded comments explaining protocol expectations and rationale for mapping to Internal.
   Clarified that h2::Reason::NO_ERROR on RST_STREAM signals missing grpc-status trailers.

2. Integration Test Coverage
   Added an integration test that simulates a truncated server response to verify the new error mapping.
   Introduced TruncatedBody and tower layer to cut off the response before trailers.
   Created missing_grpc_status_trailer_is_internal_error test ensuring client surfaces tonic::Code::Internal.
   Verified error message contains 'missing grpc-status trailer' to guard against regressions.