[apps] Add apps MCP path override (#20231)

Summary

- Add `[features.apps_mcp_path_override]` config with a `path` field for
overriding only the built-in apps MCP path.
- Keep existing host/base URL derivation unchanged and append the
configured path after that base.
- Regenerate the config schema with the custom feature-config case.

Test Plan

- Not run for latest revision; only `just fmt` and `just
write-config-schema` were run.
- Earlier revision: `cargo test -p codex-features`
- Earlier revision: `cargo test -p codex-mcp`

codex-rs/config/src/schema.rs

@@ -34,6 +34,15 @@ pub fn features_schema(schema_gen: &mut SchemaGenerator) -> Schema {
             );
             continue;
         }
+        if feature.id == codex_features::Feature::AppsMcpPathOverride {
+            validation.properties.insert(
+                feature.key.to_string(),
+                schema_gen.subschema_for::<codex_features::FeatureToml<
+                    codex_features::AppsMcpPathOverrideConfigToml,
+                >>(),
+            );
+            continue;
+        }
         validation
             .properties
             .insert(feature.key.to_string(), schema_gen.subschema_for::<bool>());