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/src/config_api.rs

@@ -17,6 +17,7 @@ use codex_core::config_loader::ConfigRequirementsToml;
 use codex_core::config_loader::LoaderOverrides;
 use codex_core::config_loader::ResidencyRequirement as CoreResidencyRequirement;
 use codex_core::config_loader::SandboxModeRequirement as CoreSandboxModeRequirement;
+use codex_protocol::config_types::WebSearchMode;
 use serde_json::json;
 use std::path::PathBuf;
 use std::sync::Arc;
@@ -115,6 +116,16 @@ fn map_requirements_toml_to_api(requirements: ConfigRequirementsToml) -> ConfigR
                 .filter_map(map_sandbox_mode_requirement_to_api)
                 .collect()
         }),
+        allowed_web_search_modes: requirements.allowed_web_search_modes.map(|modes| {
+            let mut normalized = modes
+                .into_iter()
+                .map(Into::into)
+                .collect::<Vec<WebSearchMode>>();
+            if !normalized.contains(&WebSearchMode::Disabled) {
+                normalized.push(WebSearchMode::Disabled);
+            }
+            normalized
+        }),
         enforce_residency: requirements
             .enforce_residency
             .map(map_residency_requirement_to_api),
@@ -177,6 +188,9 @@ mod tests {
                 CoreSandboxModeRequirement::ReadOnly,
                 CoreSandboxModeRequirement::ExternalSandbox,
             ]),
+            allowed_web_search_modes: Some(vec![
+                codex_core::config_loader::WebSearchModeRequirement::Cached,
+            ]),
             mcp_servers: None,
             rules: None,
             enforce_residency: Some(CoreResidencyRequirement::Us),
@@ -195,9 +209,32 @@ mod tests {
             mapped.allowed_sandbox_modes,
             Some(vec![SandboxMode::ReadOnly]),
         );
+        assert_eq!(
+            mapped.allowed_web_search_modes,
+            Some(vec![WebSearchMode::Cached, WebSearchMode::Disabled]),
+        );
         assert_eq!(
             mapped.enforce_residency,
             Some(codex_app_server_protocol::ResidencyRequirement::Us),
         );
     }
+
+    #[test]
+    fn map_requirements_toml_to_api_normalizes_allowed_web_search_modes() {
+        let requirements = ConfigRequirementsToml {
+            allowed_approval_policies: None,
+            allowed_sandbox_modes: None,
+            allowed_web_search_modes: Some(Vec::new()),
+            mcp_servers: None,
+            rules: None,
+            enforce_residency: None,
+        };
+
+        let mapped = map_requirements_toml_to_api(requirements);
+
+        assert_eq!(
+            mapped.allowed_web_search_modes,
+            Some(vec![WebSearchMode::Disabled])
+        );
+    }
 }