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-protocol/schema/json/codex_app_server_protocol.schemas.json

@@ -942,31 +942,6 @@
           "title": "Model/listRequest",
           "type": "object"
         },
-        {
-          "description": "EXPERIMENTAL - list collaboration mode presets.",
-          "properties": {
-            "id": {
-              "$ref": "#/definitions/RequestId"
-            },
-            "method": {
-              "enum": [
-                "collaborationMode/list"
-              ],
-              "title": "CollaborationMode/listRequestMethod",
-              "type": "string"
-            },
-            "params": {
-              "$ref": "#/definitions/v2/CollaborationModeListParams"
-            }
-          },
-          "required": [
-            "id",
-            "method",
-            "params"
-          ],
-          "title": "CollaborationMode/listRequest",
-          "type": "object"
-        },
         {
           "properties": {
             "id": {
@@ -5463,9 +5438,30 @@
       ],
       "type": "object"
     },
+    "InitializeCapabilities": {
+      "description": "Client-declared capabilities negotiated during initialize.",
+      "properties": {
+        "experimentalApi": {
+          "default": false,
+          "description": "Opt into receiving experimental API methods and fields.",
+          "type": "boolean"
+        }
+      },
+      "type": "object"
+    },
     "InitializeParams": {
       "$schema": "http://json-schema.org/draft-07/schema#",
       "properties": {
+        "capabilities": {
+          "anyOf": [
+            {
+              "$ref": "#/definitions/InitializeCapabilities"
+            },
+            {
+              "type": "null"
+            }
+          ]
+        },
         "clientInfo": {
           "$ref": "#/definitions/ClientInfo"
         }
@@ -10408,29 +10404,6 @@
         ],
         "type": "object"
       },
-      "CollaborationModeListParams": {
-        "$schema": "http://json-schema.org/draft-07/schema#",
-        "description": "EXPERIMENTAL - list collaboration mode presets.",
-        "title": "CollaborationModeListParams",
-        "type": "object"
-      },
-      "CollaborationModeListResponse": {
-        "$schema": "http://json-schema.org/draft-07/schema#",
-        "description": "EXPERIMENTAL - collaboration mode presets response.",
-        "properties": {
-          "data": {
-            "items": {
-              "$ref": "#/definitions/v2/CollaborationModeMask"
-            },
-            "type": "array"
-          }
-        },
-        "required": [
-          "data"
-        ],
-        "title": "CollaborationModeListResponse",
-        "type": "object"
-      },
       "CollaborationModeMask": {
         "description": "A mask for collaboration mode settings, allowing partial updates. All fields except `name` are optional, enabling selective updates.",
         "properties": {
@@ -15319,15 +15292,6 @@
               "null"
             ]
           },
-          "dynamicTools": {
-            "items": {
-              "$ref": "#/definitions/v2/DynamicToolSpec"
-            },
-            "type": [
-              "array",
-              "null"
-            ]
-          },
           "ephemeral": {
             "type": [
               "boolean",