[codex] Surface MCP reauthentication-required startup failures (#29877)

## Summary

- distinguish expired, non-refreshable stored MCP OAuth credentials from
first-time missing credentials
- carry a typed `failureReason: "reauthenticationRequired"` on the
existing `mcpServer/startupStatus/updated` notification only when user
action is required
- keep the public MCP auth-status API unchanged and regenerate the
app-server protocol schemas and documentation

## Why

An MCP server with an expired access token and no usable refresh token
currently fails startup without giving clients a reliable, typed
recovery signal.

The existing startup-status notification is the natural place to carry
this state. Its nullable `failureReason` keeps the recovery reason
attached to the failed startup transition without adding a one-off
notification. Internally, Codex distinguishes first-time login from
reauthentication and emits the reason only when the startup error itself
requires authentication.

## User impact

App clients can prompt an existing user to reconnect an MCP server when
automatic recovery is impossible by handling a failed
`mcpServer/startupStatus/updated` notification whose `failureReason` is
`reauthenticationRequired`. Starting, ready, cancelled, unrelated
failures, and first-time setup carry no reauthentication reason.

## Companion app PR

- openai/openai#1069582

## Validation

- `just test -p codex-app-server-protocol` — 248 passed; schema fixture
tests passed
- `cargo check -p codex-app-server -p codex-tui`
- `just test -p codex-rmcp-client -p codex-mcp` — 184 passed, 2 skipped
- `just test -p codex-protocol -p codex-app-server-protocol -p
codex-mcp` — 579 passed
- `just write-app-server-schema`
- `just fmt`

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

@@ -2384,6 +2384,12 @@
       ],
       "type": "object"
     },
+    "McpServerStartupFailureReason": {
+      "enum": [
+        "reauthenticationRequired"
+      ],
+      "type": "string"
+    },
     "McpServerStartupState": {
       "enum": [
         "starting",
@@ -2401,6 +2407,16 @@
             "null"
           ]
         },
+        "failureReason": {
+          "anyOf": [
+            {
+              "$ref": "#/definitions/McpServerStartupFailureReason"
+            },
+            {
+              "type": "null"
+            }
+          ]
+        },
         "name": {
           "type": "string"
         },