Configure multi_agent_v2 spawn agent hints (#17071)

Allow multi_agent_v2 features to have its own temporary configuration
under `[features.multi_agent_v2]`

```
[features.multi_agent_v2]
enabled = true
usage_hint_enabled = false
usage_hint_text = "Custom delegation guidance."
hide_spawn_agent_metadata = true
```

Absent `usage_hint_text` means use the default hint.

```
[features]
multi_agent_v2 = true
```

still works as the boolean shorthand.

codex-rs/core/src/windows_sandbox_tests.rs

@@ -109,7 +109,7 @@ fn resolve_windows_sandbox_mode_falls_back_to_legacy_keys() {
         /*value*/ true,
     );
     let cfg = ConfigToml {
-        features: Some(FeaturesToml { entries }),
+        features: Some(FeaturesToml::from(entries)),
         ..Default::default()
     };
 
@@ -127,9 +127,7 @@ fn resolve_windows_sandbox_mode_profile_legacy_false_blocks_top_level_legacy_tru
         /*value*/ false,
     );
     let profile = ConfigProfile {
-        features: Some(FeaturesToml {
-            entries: profile_entries,
-        }),
+        features: Some(FeaturesToml::from(profile_entries)),
         ..Default::default()
     };
 
@@ -139,9 +137,7 @@ fn resolve_windows_sandbox_mode_profile_legacy_false_blocks_top_level_legacy_tru
         /*value*/ true,
     );
     let cfg = ConfigToml {
-        features: Some(FeaturesToml {
-            entries: cfg_entries,
-        }),
+        features: Some(FeaturesToml::from(cfg_entries)),
         ..Default::default()
     };