[Codex] Register browser requirements feature keys (#18956)

## Summary
- register `in_app_browser` and `browser_use` as stable feature keys
- allow requirements/MDM feature requirements to pin those desktop
browser controls
- add coverage for browser requirements being accepted by config loading

## Testing
- `cargo fmt --all` (`just fmt` unavailable locally; rustfmt warned
about nightly-only `imports_granularity` config)
- `cargo test -p codex-features`
- `cargo test -p codex-core browser_feature_requirements_are_valid`
- Tested manually by setting in `requirements.toml` and seeing after app
restart state to reflect the setting was correct (at the time hiding the
`Browser Use` setting when the enterprise setting was set to false

codex-rs/core/config.schema.json

@@ -350,6 +350,9 @@
             "apps": {
               "type": "boolean"
             },
+            "browser_use": {
+              "type": "boolean"
+            },
             "child_agents_md": {
               "type": "boolean"
             },
@@ -422,6 +425,9 @@
             "image_generation": {
               "type": "boolean"
             },
+            "in_app_browser": {
+              "type": "boolean"
+            },
             "include_apply_patch_tool": {
               "type": "boolean"
             },
@@ -2362,6 +2368,9 @@
         "apps": {
           "type": "boolean"
         },
+        "browser_use": {
+          "type": "boolean"
+        },
         "child_agents_md": {
           "type": "boolean"
         },
@@ -2434,6 +2443,9 @@
         "image_generation": {
           "type": "boolean"
         },
+        "in_app_browser": {
+          "type": "boolean"
+        },
         "include_apply_patch_tool": {
           "type": "boolean"
         },