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/GetAccountResponse.json

@@ -0,0 +1,83 @@
+{
+  "$schema": "http://json-schema.org/draft-07/schema#",
+  "definitions": {
+    "Account": {
+      "oneOf": [
+        {
+          "properties": {
+            "type": {
+              "enum": [
+                "apiKey"
+              ],
+              "title": "ApiKeyAccountType",
+              "type": "string"
+            }
+          },
+          "required": [
+            "type"
+          ],
+          "title": "ApiKeyAccount",
+          "type": "object"
+        },
+        {
+          "properties": {
+            "email": {
+              "type": "string"
+            },
+            "planType": {
+              "$ref": "#/definitions/PlanType"
+            },
+            "type": {
+              "enum": [
+                "chatgpt"
+              ],
+              "title": "ChatgptAccountType",
+              "type": "string"
+            }
+          },
+          "required": [
+            "email",
+            "planType",
+            "type"
+          ],
+          "title": "ChatgptAccount",
+          "type": "object"
+        }
+      ]
+    },
+    "PlanType": {
+      "enum": [
+        "free",
+        "go",
+        "plus",
+        "pro",
+        "team",
+        "business",
+        "enterprise",
+        "edu",
+        "unknown"
+      ],
+      "type": "string"
+    }
+  },
+  "properties": {
+    "account": {
+      "anyOf": [
+        {
+          "$ref": "#/definitions/Account"
+        },
+        {
+          "type": "null"
+        }
+      ]
+    },
+    "requiresOpenaiAuth": {
+      "type": "boolean"
+    }
+  },
+  "required": [
+    "requiresOpenaiAuth"
+  ],
+  "title": "GetAccountResponse",
+  "type": "object"
+}