feat(coding-agent): accept native extension providers

Allow extensions to register complete pi-ai Provider objects while preserving models.json composition and resolved provider auth access.

Refs #4823 and #4824.

packages/coding-agent/src/core/agent-session.ts

@@ -2419,6 +2419,10 @@ export class AgentSession {
 					this._modelRuntime.registerProvider(name, config);
 					this._refreshCurrentModelFromRegistry();
 				},
+				registerNativeProvider: (provider) => {
+					this._modelRuntime.registerNativeProvider(provider);
+					this._refreshCurrentModelFromRegistry();
+				},
 				unregisterProvider: (name) => {
 					this._modelRuntime.unregisterProvider(name);
 					this._refreshCurrentModelFromRegistry();