feat: add support for allowed_web_search_modes in requirements.toml (#10964)

This PR makes it possible to disable live web search via an enterprise
config even if the user is running in `--yolo` mode (though cached web
search will still be available). To do this, create
`/etc/codex/requirements.toml` as follows:

```toml
# "live" is not allowed; "disabled" is allowed even though not listed explicitly.
allowed_web_search_modes = ["cached"]
```

Or set `requirements_toml_base64` MDM as explained on
https://developers.openai.com/codex/security/#locations.

### Why
- Enforce admin/MDM/`requirements.toml` constraints on web-search
behavior, independent of user config and per-turn sandbox defaults.
- Ensure per-turn config resolution and review-mode overrides never
crash when constraints are present.

### What
- Add `allowed_web_search_modes` to requirements parsing and surface it
in app-server v2 `ConfigRequirements` (`allowedWebSearchModes`), with
fixtures updated.
- Define a requirements allowlist type (`WebSearchModeRequirement`) and
normalize semantics:
  - `disabled` is always implicitly allowed (even if not listed).
  - An empty list is treated as `["disabled"]`.
- Make `Config.web_search_mode` a `Constrained<WebSearchMode>` and apply
requirements via `ConstrainedWithSource<WebSearchMode>`.
- Update per-turn resolution (`resolve_web_search_mode_for_turn`) to:
- Prefer `Live → Cached → Disabled` when
`SandboxPolicy::DangerFullAccess` is active (subject to requirements),
unless the user preference is explicitly `Disabled`.
- Otherwise, honor the user’s preferred mode, falling back to an allowed
mode when necessary.
- Update TUI `/debug-config` and app-server mapping to display
normalized `allowed_web_search_modes` (including implicit `disabled`).
- Fix web-search integration tests to assert cached behavior under
`SandboxPolicy::ReadOnly` (since `DangerFullAccess` legitimately prefers
`live` when allowed).

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

@@ -30,6 +30,15 @@
             "null"
           ]
         },
+        "allowedWebSearchModes": {
+          "items": {
+            "$ref": "#/definitions/WebSearchMode"
+          },
+          "type": [
+            "array",
+            "null"
+          ]
+        },
         "enforceResidency": {
           "anyOf": [
             {
@@ -56,6 +65,14 @@
         "danger-full-access"
       ],
       "type": "string"
+    },
+    "WebSearchMode": {
+      "enum": [
+        "disabled",
+        "cached",
+        "live"
+      ],
+      "type": "string"
     }
   },
   "properties": {