feat: vendor app-server protocol schema fixtures (#10371)

Similar to what @sayan-oai did in openai/codex#8956 for
`config.schema.json`, this PR updates the repo so that it includes the
output of `codex app-server generate-json-schema` and `codex app-server
generate-ts` and adds a test to verify it is in sync with the current
code.

Motivation:
- This makes any schema changes introduced by a PR transparent during
code review.
- In particular, this should help us catch PRs that would introduce a
non-backwards-compatible change to the app schema (eventually, this
should also be enforced by tooling).
- Once https://github.com/openai/codex/pull/10231 is in to formalize the
notion of "experimental" fields, we can work on ensuring the
non-experimental bits are backwards-compatible.

`codex-rs/app-server-protocol/tests/schema_fixtures.rs` was added as the
test and `just write-app-server-schema` can be use to generate the
vendored schema files.

Incidentally, when I run:

```
rg _ codex-rs/app-server-protocol/schema/typescript/v2
```

I see a number of `snake_case` names that should be `camelCase`.

codex-rs/app-server-protocol/schema/json/v2/ThreadForkParams.json

@@ -0,0 +1,98 @@
+{
+  "$schema": "http://json-schema.org/draft-07/schema#",
+  "definitions": {
+    "AskForApproval": {
+      "enum": [
+        "untrusted",
+        "on-failure",
+        "on-request",
+        "never"
+      ],
+      "type": "string"
+    },
+    "SandboxMode": {
+      "enum": [
+        "read-only",
+        "workspace-write",
+        "danger-full-access"
+      ],
+      "type": "string"
+    }
+  },
+  "description": "There are two ways to fork a thread: 1. By thread_id: load the thread from disk by thread_id and fork it into a new thread. 2. By path: load the thread from disk by path and fork it into a new thread.\n\nIf using path, the thread_id param will be ignored.\n\nPrefer using thread_id whenever possible.",
+  "properties": {
+    "approvalPolicy": {
+      "anyOf": [
+        {
+          "$ref": "#/definitions/AskForApproval"
+        },
+        {
+          "type": "null"
+        }
+      ]
+    },
+    "baseInstructions": {
+      "type": [
+        "string",
+        "null"
+      ]
+    },
+    "config": {
+      "additionalProperties": true,
+      "type": [
+        "object",
+        "null"
+      ]
+    },
+    "cwd": {
+      "type": [
+        "string",
+        "null"
+      ]
+    },
+    "developerInstructions": {
+      "type": [
+        "string",
+        "null"
+      ]
+    },
+    "model": {
+      "description": "Configuration overrides for the forked thread, if any.",
+      "type": [
+        "string",
+        "null"
+      ]
+    },
+    "modelProvider": {
+      "type": [
+        "string",
+        "null"
+      ]
+    },
+    "path": {
+      "description": "[UNSTABLE] Specify the rollout path to fork from. If specified, the thread_id param will be ignored.",
+      "type": [
+        "string",
+        "null"
+      ]
+    },
+    "sandbox": {
+      "anyOf": [
+        {
+          "$ref": "#/definitions/SandboxMode"
+        },
+        {
+          "type": "null"
+        }
+      ]
+    },
+    "threadId": {
+      "type": "string"
+    }
+  },
+  "required": [
+    "threadId"
+  ],
+  "title": "ThreadForkParams",
+  "type": "object"
+}