fix(coding): recover provider context failures

tests/unit/coding-core-routes.test.ts

@@ -30,6 +30,7 @@ import {
 import type {
   ConversationSnapshot,
   ConversationPatchEnvelope,
+  PrepareConversationInput,
   PromptConversationInput,
 } from '../../electron/coding-runtime/contracts';
 import { archivePiConversationSession } from '../../electron/coding-runtime/pi/resource-loader';
@@ -175,7 +176,22 @@ describe('PI-100 coding core Host contract', () => {
         mission: 'Implement', owns: [], boundaries: [], collaborators: [], principles: [],
       },
     });
-    const runtime = new InMemoryConversationRuntime();
+    class StrictResolvedModelRuntime extends InMemoryConversationRuntime {
+      readonly prepareInputs: PrepareConversationInput[] = [];
+
+      override async prepare(input: PrepareConversationInput) {
+        this.prepareInputs.push(structuredClone(input));
+        if (input.model.modelResolution !== 'resolved' || !input.model.model) {
+          throw new CodingRuntimeContractError(
+            'CODING_MIGRATION_MODEL_REQUIRED',
+            'A resolved model is required before preparation',
+            true,
+          );
+        }
+        return await super.prepare(input);
+      }
+    }
+    const runtime = new StrictResolvedModelRuntime();
     const projects = new CodingProjectService(store);
     const conversations = new CodingConversationService(projects, runtime);
     const conversation = await conversations.createConversation({ agentId: 'builder', title: 'Resolve me' });
@@ -189,6 +205,10 @@ describe('PI-100 coding core Host contract', () => {
       conversationId: conversation.id,
       model: { model: MODEL, modelResolution: 'resolved' },
     }));
+    expect(runtime.prepareInputs).toHaveLength(1);
+    expect(runtime.prepareInputs.every(({ model }) => (
+      model.modelResolution === 'resolved' && model.model !== null
+    ))).toBe(true);
     await expect(projects.conversationStore(root).get(conversation.id)).resolves.toMatchObject({
       model: MODEL,
       modelResolution: 'resolved',