fix(plugins): keep empty session extension pins authoritative

Pinned session-extension registries now remain the owner even when empty, preventing later active registry churn from leaking agent-owned extensions into the gateway surface.

src/plugins/runtime.test.ts

@@ -198,7 +198,7 @@ describe("plugin runtime route registry", () => {
     expect(isPluginRegistryRetired(startupRegistry)).toBe(true);
   });
 
-  it("falls through from an empty pinned session extension registry to the active surface", () => {
+  it("keeps empty pinned session extension registries authoritative over active extensions", () => {
     const startupRegistry = createEmptyPluginRegistry();
     const laterRegistry = createRegistryWithSessionExtension("later", "presence");
 
@@ -207,8 +207,9 @@ describe("plugin runtime route registry", () => {
     const sessionExtensionVersionBeforeSwap = getActivePluginSessionExtensionRegistryVersion();
     setActivePluginRegistry(laterRegistry);
 
-    expect(getActivePluginSessionExtensionRegistry()).toBe(laterRegistry);
-    expect(getActivePluginSessionExtensionRegistryVersion()).not.toBe(
+    expect(getActivePluginSessionExtensionRegistry()).toBe(startupRegistry);
+    expect(getActivePluginSessionExtensionRegistry()?.sessionExtensions).toHaveLength(0);
+    expect(getActivePluginSessionExtensionRegistryVersion()).toBe(
       sessionExtensionVersionBeforeSwap,
     );
   });

src/plugins/runtime.ts

@@ -353,10 +353,6 @@ export function requireActivePluginChannelRegistry(): PluginRegistry {
   return created;
 }
 
-function countSessionExtensionSurface(registry: PluginRegistry | null): number {
-  return registry?.sessionExtensions?.length ?? 0;
-}
-
 export function pinActivePluginSessionExtensionRegistry(registry: PluginRegistry) {
   const previousRegistry = asPluginRegistry(state.sessionExtension.registry);
   installSurfaceRegistry(state.sessionExtension, registry, true);
@@ -380,15 +376,7 @@ export function releasePinnedPluginSessionExtensionRegistry(registry?: PluginReg
 }
 
 export function getActivePluginSessionExtensionRegistry(): PluginRegistry | null {
-  const surfaceRegistry = asPluginRegistry(state.sessionExtension.registry ?? state.activeRegistry);
-  if (!state.sessionExtension.pinned) {
-    return surfaceRegistry;
-  }
-  const activeRegistry = asPluginRegistry(state.activeRegistry);
-  return countSessionExtensionSurface(surfaceRegistry) > 0 ||
-    countSessionExtensionSurface(activeRegistry) === 0
-    ? surfaceRegistry
-    : activeRegistry;
+  return asPluginRegistry(state.sessionExtension.registry ?? state.activeRegistry);
 }
 
 export function getActivePluginSessionExtensionRegistryVersion(): number {