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/protocol/src/protocol.rs

@@ -3,6 +3,7 @@
 //! Uses a SQ (Submission Queue) / EQ (Event Queue) pattern to asynchronously communicate
 //! between user and agent.
 
+use std::collections::BTreeMap;
 use std::collections::HashMap;
 use std::fmt;
 use std::ops::Mul;
@@ -471,6 +472,21 @@ pub struct ThreadSettingsOverrides {
     pub personality: Option<Personality>,
 }
 
+/// Source classification for client-supplied context.
+#[derive(Debug, Clone, Copy, Deserialize, Serialize, PartialEq, Eq, JsonSchema)]
+#[serde(rename_all = "snake_case")]
+pub enum AdditionalContextKind {
+    Untrusted,
+    Application,
+}
+
+/// Client-supplied context keyed by an opaque source identifier.
+#[derive(Debug, Clone, Deserialize, Serialize, PartialEq, Eq, JsonSchema)]
+pub struct AdditionalContextEntry {
+    pub value: String,
+    pub kind: AdditionalContextKind,
+}
+
 /// Submission operation
 #[derive(Debug, Clone, Deserialize, Serialize, PartialEq, JsonSchema)]
 #[serde(tag = "type", rename_all = "snake_case")]
@@ -513,6 +529,9 @@ pub enum Op {
         /// Optional turn-scoped Responses API `client_metadata`.
         #[serde(default, skip_serializing_if = "Option::is_none")]
         responsesapi_client_metadata: Option<HashMap<String, String>>,
+        /// Client-supplied context fragments keyed by an opaque source identifier.
+        #[serde(default, skip_serializing_if = "BTreeMap::is_empty")]
+        additional_context: BTreeMap<String, AdditionalContextEntry>,
 
         /// Persistent thread-settings overrides to apply before the input.
         #[serde(default, flatten)]
@@ -655,6 +674,7 @@ impl From<Vec<UserInput>> for Op {
             items: value,
             final_output_json_schema: None,
             responsesapi_client_metadata: None,
+            additional_context: Default::default(),
             thread_settings: ThreadSettingsOverrides::default(),
         }
     }
@@ -4954,6 +4974,7 @@ mod tests {
             items: Vec::new(),
             final_output_json_schema: None,
             responsesapi_client_metadata: None,
+            additional_context: Default::default(),
             thread_settings: Default::default(),
         };
 
@@ -4974,6 +4995,7 @@ mod tests {
                 items: Vec::new(),
                 final_output_json_schema: None,
                 responsesapi_client_metadata: None,
+                additional_context: Default::default(),
                 thread_settings: Default::default(),
             }
         );
@@ -4996,6 +5018,7 @@ mod tests {
             items: Vec::new(),
             final_output_json_schema: Some(schema.clone()),
             responsesapi_client_metadata: None,
+            additional_context: Default::default(),
             thread_settings: Default::default(),
         };
 
@@ -5022,6 +5045,7 @@ mod tests {
                 "fiber_run_id".to_string(),
                 "fiber-123".to_string(),
             )])),
+            additional_context: Default::default(),
             thread_settings: Default::default(),
         };