[Docs] Unwrap the lone Rust tab in the streaming-events article

The "Subscribing before a session starts" section has only a Rust
example, and the docs normalization pipeline converts a `<details>` group
into a tabbed language switcher only when two or more consecutive blocks
are present. A single block renders as raw collapsible HTML on
docs.github.com.

Drop the `<details>`/`<summary>` wrapper and leave the code fence
directly in the article, matching the repository docs style guide.

Co-authored-by: Copilot App <223556219+Copilot@users.noreply.github.com>
Copilot-Session: 0f9d5ac7-9999-4f37-82b3-a5533bfbc1f6

Author: jmoseley

docs/features/streaming-events.md

@@ -225,9 +225,6 @@ A session can emit events before its create or resume call returns. The agent ma
 > [!TIP]
 > **(Rust)** `Client::prepare_session` and `Client::prepare_resume_session` return a `PreparedSession` that owns the session's event channel before any protocol activity happens. Subscribe first, then call `start()`.
 
-<details open>
-<summary><strong>Rust</strong></summary>
-
 ```rust
 use github_copilot_sdk::{Client, SessionConfig};
 
@@ -252,8 +249,6 @@ async fn create_without_missing_startup_events(
 }
 ```
 
-</details>
-
 `prepare_*` is synchronous and inert: it validates the buffer capacity, allocates a local channel, and does nothing else. No session is registered and nothing reaches the CLI until `start()` is first polled. Dropping a prepared session that was never started leaves no state behind and closes its subscriptions; dropping the `start()` future cancels the in-flight startup and unregisters the session, so a retry with the same session ID succeeds. Cleanup is scoped to the exact registration the abandoned startup owned, so it cannot evict a retry that has already taken over the same session ID.
 
 Startup buffering is worth planning for: