Server-streaming requests block until first message is received

## Bug Report

### Version

tonic v0.13.1
tonic-build v0.13.1

### Platform

`Linux arbiter 6.14.10-zen1-1-zen #1 ZEN SMP PREEMPT_DYNAMIC Wed, 04 Jun 2025 18:52:21 +0000 x86_64 GNU/Linux`

### Description

Upon requesting a streaming response, the (let's call it) "initial" call blocks until the first message is received, rather than `.next()` doing the blocking.

My code goes something like this :
```rust
let mut response = client.streaming_request(request.clone()).await?.into_inner();
while let Some(obj) = response.next().await { ... }
```

I would expect `.next()` to do *all* the blocking here (bar establishing the connection), but the blockage is split across `.streaming_request()` *and* `.next()`, the former blocking until the first streamed response, and the latter handling all subsequent responses.