## Overview

Two tests added in #1254 were failing on `main`. The Rust "Test Suite"
CI job is not a required check (only `devflow/mergegate` is), so the red
suite did not block #1254 from merging.

### 1.
`enrich_ctx_keeps_tracer_set_cold_start_trace_id_without_tracer_detected`
Annotated `#[test]` but calls `setup()`, which spawns a Tokio task via
`tokio::spawn(service.run())`. Running it panicked with `there is no
reactor running, must be called from the context of a Tokio 1.x
runtime`. Changed to `#[tokio::test] async fn` to match its sibling
tests.

### 2. `timeout_without_tracer_does_not_send_cold_start_span`
Passed its `trace_sender` by value into `on_platform_runtime_done`,
dropping the only `Arc<SendingTraceProcessor>` before `try_recv()`. The
receiver then reported `Disconnected`, but the test asserted `Empty`, so
it failed even though the production code correctly never sent the cold
start span. Fixed by passing a clone and holding the sender until after
the assertion so the channel stays connected.

No production code changes; both fixes are test-only.

> *"A test that drops its own sender and then complains the line went
dead: the unit-testing equivalent of cutting the phone cord and filing a
complaint about the silence."* — Claude 🤖