feat: add support for allowed_web_search_modes in requirements.toml

Example:

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

### Why
- Support admin/MDM/requirements.toml constraints over web-search behavior, independent of user config or sandbox defaults.
- Ensure per-turn config resolution and review-mode overrides can never crash when constraints are present.
- Note that `allowed_web_search_modes = ["cached"]` denies `"live"`, even if `--yolo` is used. See `resolve_web_search_mode_for_turn()`.

### What
- Add `allowed_web_search_modes` to requirements parsing and app-server v2 `ConfigRequirements` (`allowedWebSearchModes`), with schema/TS fixture updates.
- Introduce TOML-only `WebSearchModeRequirement` for requirements allowlists; accept `disabled|cached|live`, and treat an empty list as `[disabled]`.
- Convert the allowlist to a `ConstrainedWithSource<Option<WebSearchMode>>`, always permitting `Disabled` while enforcing membership for other values.
- Make `Config.web_search_mode` a constrained field, update call sites/tests, and surface constraint violations via warnings/fallback.
- Extend TUI `/debug-config` output to display `allowed_web_search_modes`.

### Safety
- Avoid `expect()` on constrained `web_search_mode` mutation in session per-turn config and review-thread setup; warn and keep the constrained value instead.

codex-rs/app-server-protocol/schema/json/codex_app_server_protocol.schemas.json

@@ -10988,6 +10988,15 @@
               "null"
             ]
           },
+          "allowedWebSearchModes": {
+            "items": {
+              "$ref": "#/definitions/v2/WebSearchMode"
+            },
+            "type": [
+              "array",
+              "null"
+            ]
+          },
           "enforceResidency": {
             "anyOf": [
               {