Unary gRPC drops trace-context propagation when client tracing is disabled

Unconfirmed. This is the result of a search with Claude, may be a false positive.

In `src/gax-internal/src/grpc.rs`, unary gRPC requests inject distributed trace context into outbound metadata headers:

https://github.com/googleapis/google-cloud-rust/blob/f02b42c2ac5720334cd2b5335824bd4166264527/src/gax-internal/src/grpc.rs#L496-L506

The injected span is created by `grpc_helpers::unary_make_request_span`. When `self.tracing_attributes` is `None` (client telemetry tracing disabled), `unary_make_request_span` returns `tracing::Span::none()`:

https://github.com/googleapis/google-cloud-rust/blob/f02b42c2ac5720334cd2b5335824bd4166264527/src/gax-internal/src/grpc/grpc_helpers.rs#L142-L144

Injecting from `Span::none()` sends no traceparent or baggage headers. In contrast, all other transport paths (HTTP at `http.rs:326-331`, `501-506`, and gRPC streaming at `grpc.rs:212`, `367`) inject from `&tracing::Span::current()` to propagate active trace context regardless of whether client tracing is enabled.

Start in src/gax-internal/src/grpc.rs at the unary request metadata injection and read grpc_helpers::unary_make_request_span in src/gax-internal/src/grpc/grpc_helpers.rs. Compare these paths with the HTTP locations in http.rs and streaming injections in grpc.rs. Done means unary requests preserve active trace context when client tracing is disabled.

Written by the indexing model from the issue text.