Add experimental turn additional context (#24154)

## Summary

Adds experimental `additionalContext` support to `turn/start` and
`turn/steer` so clients can provide ephemeral external context, such as
browser or automation state, without turning that plumbing into a
visible user prompt or triggering user-prompt lifecycle behavior.

## API Shape

The parameter shape is:

```ts
additionalContext?: Record<string, {
  value: string
  kind: "untrusted" | "application"
}> | null
```

Example:

```json
{
  "additionalContext": {
    "browser_info": {
      "value": "Active tab is CI failures.",
      "kind": "untrusted"
    },
    "automation_info": {
      "value": "CI rerun is in progress.",
      "kind": "application"
    }
  }
}
```

The keys are opaque and caller-defined.

## Context Injection

When provided, accepted entries are inserted into model context as
hidden contextual message items, not as visible thread user-message
items.

`kind: "untrusted"` entries are inserted with role `user`:

```text
<external_${key}>${value}</external_${key}>
```

`kind: "application"` entries are inserted with role `developer`:

```text
<${key}>${value}</${key}>
```

Values are not escaped. Each value is truncated to 1k approximate tokens
before wrapping.

For `turn/start`, accepted additional context is inserted before normal
user input. For `turn/steer`, additional context is merged only when the
steer includes non-empty user input; context-only steers still reject as
empty input.

## Dedupe Strategy

`AdditionalContextStore` lives on session state and stores the latest
complete additional-context map.

Each `turn/start` or non-empty `turn/steer` treats its
`additionalContext` as the current complete set of values. Entries are
injected only when the key is new or the exact entry for that key
changed, including `value` or `kind`. After merging, the store is
replaced with the provided map, so omitted keys are removed from the
retained set and can be injected again later if reintroduced.

Omitting `additionalContext`, passing `null`, or passing an empty object
resets the store to empty and injects nothing.

## What Changed

- Threads experimental v2 `additionalContext` through app-server into
core turn start and steer handling.
- Adds separate contextual fragment types for untrusted user-role
context and application developer-role context.
- Uses pending response input items so additional context can be
combined with normal user input without treating it as prompt text.
- Adds integration coverage for start/steer flow, role routing,
dedupe/reset behavior, deletion/re-add behavior, hook-blocked input
behavior, empty context-only steer rejection, external-fragment marker
matching, and truncation.

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

@@ -3428,6 +3428,7 @@ fn turn_start_params_preserve_explicit_null_service_tier() {
         thread_id: "thread_123".to_string(),
         input: vec![],
         responsesapi_client_metadata: None,
+        additional_context: None,
         environments: None,
         cwd: None,
         runtime_workspace_roots: None,

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

@@ -41,6 +41,23 @@ pub struct TurnEnvironmentParams {
     pub cwd: AbsolutePathBuf,
 }
 
+#[derive(Serialize, Deserialize, Debug, Clone, Copy, PartialEq, Eq, JsonSchema, TS)]
+#[serde(rename_all = "lowercase")]
+#[ts(rename_all = "lowercase")]
+#[ts(export_to = "v2/")]
+pub enum AdditionalContextKind {
+    Untrusted,
+    Application,
+}
+
+#[derive(Serialize, Deserialize, Debug, Clone, PartialEq, Eq, JsonSchema, TS)]
+#[serde(rename_all = "camelCase")]
+#[ts(export_to = "v2/")]
+pub struct AdditionalContextEntry {
+    pub value: String,
+    pub kind: AdditionalContextKind,
+}
+
 #[derive(
     Serialize, Deserialize, Debug, Default, Clone, PartialEq, JsonSchema, TS, ExperimentalApi,
 )]
@@ -53,6 +70,10 @@ pub struct TurnStartParams {
     #[experimental("turn/start.responsesapiClientMetadata")]
     #[ts(optional = nullable)]
     pub responsesapi_client_metadata: Option<HashMap<String, String>>,
+    /// Optional client-provided context fragments keyed by an opaque source identifier.
+    #[experimental("turn/start.additionalContext")]
+    #[ts(optional = nullable)]
+    pub additional_context: Option<HashMap<String, AdditionalContextEntry>>,
     /// Optional turn-scoped environments.
     ///
     /// Omitted uses the thread sticky environments. Empty disables
@@ -141,6 +162,10 @@ pub struct TurnSteerParams {
     #[experimental("turn/steer.responsesapiClientMetadata")]
     #[ts(optional = nullable)]
     pub responsesapi_client_metadata: Option<HashMap<String, String>>,
+    /// Optional client-provided context fragments keyed by an opaque source identifier.
+    #[experimental("turn/steer.additionalContext")]
+    #[ts(optional = nullable)]
+    pub additional_context: Option<HashMap<String, AdditionalContextEntry>>,
     /// Required active turn id precondition. The request fails when it does not
     /// match the currently active turn.
     pub expected_turn_id: String,