Add optional names to untrusted additional context (#57181)

## What changed

- Add an optional `name` to `AdditionalContextEntry` across the protocol, app-server schemas, and generated clients.
- Accept names matching `[a-z][a-z0-9_]{0,63}` only for `untrusted` context. Reject invalid names and names on `application` context before admitting turn input, returning invalid-request errors from `turn/start` and `turn/steer`.
- Preserve existing model-input rendering and deduplicate context when only its name changes.

## Testing

Add coverage for valid name boundaries, callers that omit `name`, invalid-name rejection without admitting input, and deduplication across name changes.

GitOrigin-RevId: 2caffa6fddd40ddb178733b335f520f317239edd

codex-rs/app-server-protocol/src/protocol/v2/turn.rs

@@ -116,6 +116,9 @@ pub enum AdditionalContextKind {
 pub struct AdditionalContextEntry {
     pub value: String,
     pub kind: AdditionalContextKind,
+    /// Optional name for untrusted context, used to label its tool output.
+    #[ts(optional = nullable)]
+    pub name: Option<String>,
 }
 
 /// Requested cyber treatment for an OpenAI model turn.