feat: experimental flags (#10231)

## Problem being solved
- We need a single, reliable way to mark app-server API surface as
experimental so that:
  1. the runtime can reject experimental usage unless the client opts in
2. generated TS/JSON schemas can exclude experimental methods/fields for
stable clients.

Right now that’s easy to drift or miss when done ad-hoc.

## How to declare experimental methods and fields
- **Experimental method**: add `#[experimental("method/name")]` to the
`ClientRequest` variant in `client_request_definitions!`.
- **Experimental field**: on the params struct, derive `ExperimentalApi`
and annotate the field with `#[experimental("method/name.field")]` + set
`inspect_params: true` for the method variant so
`ClientRequest::experimental_reason()` inspects params for experimental
fields.

## How the macro solves it
- The new derive macro lives in
`codex-rs/codex-experimental-api-macros/src/lib.rs` and is used via
`#[derive(ExperimentalApi)]` plus `#[experimental("reason")]`
attributes.
- **Structs**:
- Generates `ExperimentalApi::experimental_reason(&self)` that checks
only annotated fields.
  - The “presence” check is type-aware:
    - `Option<T>`: `is_some_and(...)` recursively checks inner.
    - `Vec`/`HashMap`/`BTreeMap`: must be non-empty.
    - `bool`: must be `true`.
    - Other types: considered present (returns `true`).
- Registers each experimental field in an `inventory` with `(type_name,
serialized field name, reason)` and exposes `EXPERIMENTAL_FIELDS` for
that type. Field names are converted from `snake_case` to `camelCase`
for schema/TS filtering.
- **Enums**:
- Generates an exhaustive `match` returning `Some(reason)` for annotated
variants and `None` otherwise (no wildcard arm).
- **Wiring**:
- Runtime gating uses `ExperimentalApi::experimental_reason()` in
`codex-rs/app-server/src/message_processor.rs` to reject requests unless
`InitializeParams.capabilities.experimental_api == true`.
- Schema/TS export filters use the inventory list and
`EXPERIMENTAL_CLIENT_METHODS` from `client_request_definitions!` to
strip experimental methods/fields when `experimental_api` is false.

codex-rs/app-server/src/codex_message_processor.rs

@@ -69,6 +69,8 @@ use codex_app_server_protocol::McpServerOauthLoginParams;
 use codex_app_server_protocol::McpServerOauthLoginResponse;
 use codex_app_server_protocol::McpServerRefreshResponse;
 use codex_app_server_protocol::McpServerStatus;
+use codex_app_server_protocol::MockExperimentalMethodParams;
+use codex_app_server_protocol::MockExperimentalMethodResponse;
 use codex_app_server_protocol::ModelListParams;
 use codex_app_server_protocol::ModelListResponse;
 use codex_app_server_protocol::NewConversationParams;
@@ -507,6 +509,9 @@ impl CodexMessageProcessor {
                         .await;
                 });
             }
+            ClientRequest::MockExperimentalMethod { request_id, params } => {
+                self.mock_experimental_method(request_id, params).await;
+            }
             ClientRequest::McpServerOauthLogin { request_id, params } => {
                 self.mcp_server_oauth_login(request_id, params).await;
             }
@@ -1606,6 +1611,7 @@ impl CodexMessageProcessor {
             base_instructions,
             developer_instructions,
             dynamic_tools,
+            mock_experimental_field: _mock_experimental_field,
             experimental_raw_events,
             personality,
             ephemeral,
@@ -3001,6 +3007,16 @@ impl CodexMessageProcessor {
         outgoing.send_response(request_id, response).await;
     }
 
+    async fn mock_experimental_method(
+        &self,
+        request_id: RequestId,
+        params: MockExperimentalMethodParams,
+    ) {
+        let MockExperimentalMethodParams { value } = params;
+        let response = MockExperimentalMethodResponse { echoed: value };
+        self.outgoing.send_response(request_id, response).await;
+    }
+
     async fn mcp_server_refresh(&self, request_id: RequestId, _params: Option<()>) {
         let config = match self.load_latest_config().await {
             Ok(config) => config,