feat(app-server): expose rate-limit reset credits (#28143)

## Why

Codex users can earn personal rate-limit reset credits, but app-server
clients do not currently have an API for reading or redeeming them. This
adds the backend and protocol foundation used by the `/usage` TUI flow
in #28154.

## What changed

- Extend `account/rateLimits/read` with a nullable
`rateLimitResetCredits` summary sourced from the existing usage
response.
- Add backend-client and app-server support for consuming a reset with a
caller-generated idempotency key. A UUID is recommended, and clients
reuse the same key when retrying the same logical reset.
- Return only the consume `outcome`; clients refetch
`account/rateLimits/read` for updated window state.
- Document the response field and each consume outcome, and regenerate
the JSON and TypeScript schema fixtures.
- Clarify in `AGENTS.md` that new app-server string enum values use
camelCase on the wire.
- Update the existing TUI response fixture for the expanded protocol
shape.
- Add coverage for authentication, response mapping, backend failures,
consume outcomes, and request timeout behavior.

## Validation

- `just test -p codex-app-server-protocol` — 231 passed.
- `just test -p codex-backend-client` — 14 passed.
- Focused `codex-app-server` reset-credit tests — 5 passed.
- Focused `codex-tui` protocol response fixture test — passed.
- `just fix -p codex-backend-client -p codex-app-server-protocol -p
codex-app-server` — passed.
- `just fmt` — passed.

codex-rs/app-server-protocol/schema/json/ClientRequest.json

@@ -554,6 +554,18 @@
       ],
       "type": "object"
     },
+    "ConsumeAccountRateLimitResetCreditParams": {
+      "properties": {
+        "idempotencyKey": {
+          "description": "Identifies one logical reset attempt. A UUID is recommended; reuse the same value when retrying that attempt.",
+          "type": "string"
+        }
+      },
+      "required": [
+        "idempotencyKey"
+      ],
+      "type": "object"
+    },
     "ContentItem": {
       "oneOf": [
         {
@@ -6179,6 +6191,30 @@
       "title": "Account/rateLimits/readRequest",
       "type": "object"
     },
+    {
+      "properties": {
+        "id": {
+          "$ref": "#/definitions/RequestId"
+        },
+        "method": {
+          "enum": [
+            "account/rateLimitResetCredit/consume"
+          ],
+          "title": "Account/rateLimitResetCredit/consumeRequestMethod",
+          "type": "string"
+        },
+        "params": {
+          "$ref": "#/definitions/ConsumeAccountRateLimitResetCreditParams"
+        }
+      },
+      "required": [
+        "id",
+        "method",
+        "params"
+      ],
+      "title": "Account/rateLimitResetCredit/consumeRequest",
+      "type": "object"
+    },
     {
       "properties": {
         "id": {