feat(FN-5204): add test mode activation across settings, engine, and dashboard

- Add core settings schema/types support for testMode with model-resolution override handling
- Enforce engine session lane overrides in test mode with targeted helper coverage
- Add dashboard settings toggle plus persistent test-mode banner and related component tests
- Update settings documentation and parity/roundtrip tests for the new test mode behavior

packages/engine/src/__tests__/agent-session-helpers-test-mode.test.ts

@@ -0,0 +1,111 @@
+import { describe, expect, it } from "vitest";
+import {
+  resolveExecutorSessionModel,
+  resolveHeartbeatSessionModels,
+  resolveMergerSessionModel,
+  resolvePlanningSessionModel,
+} from "../agent-session-helpers.js";
+
+const assignedAgentRuntimeConfig = {
+  modelProvider: "anthropic",
+  modelId: "claude-sonnet-4-5",
+};
+
+describe("agent-session-helpers test mode overrides", () => {
+  it("forces every resolver lane to mock when settings.testMode is true", () => {
+    const settings = {
+      testMode: true,
+      defaultProvider: "openai",
+      defaultModelId: "gpt-4.1",
+      executionProvider: "openai",
+      executionModelId: "gpt-4.1",
+      planningProvider: "openai",
+      planningModelId: "gpt-4.1",
+      defaultProviderOverride: "openai",
+      defaultModelIdOverride: "gpt-4.1",
+    };
+
+    expect(resolveExecutorSessionModel("openai", "gpt-4.1", settings, assignedAgentRuntimeConfig)).toEqual({
+      provider: "mock",
+      modelId: "scripted",
+    });
+    expect(resolvePlanningSessionModel("openai", "gpt-4.1", settings, assignedAgentRuntimeConfig)).toEqual({
+      provider: "mock",
+      modelId: "scripted",
+    });
+    expect(resolveMergerSessionModel(settings, assignedAgentRuntimeConfig)).toEqual({
+      provider: "mock",
+      modelId: "scripted",
+    });
+    expect(resolveHeartbeatSessionModels(settings, assignedAgentRuntimeConfig)).toEqual({
+      defaultProvider: "mock",
+      defaultModelId: "scripted",
+      fallbackProvider: undefined,
+      fallbackModelId: undefined,
+    });
+  });
+
+  it("forces every resolver lane to mock when defaultProvider is mock", () => {
+    const settings = {
+      defaultProvider: "mock",
+      defaultModelId: "custom",
+      executionProvider: "openai",
+      executionModelId: "gpt-4.1",
+      planningProvider: "openai",
+      planningModelId: "gpt-4.1",
+      defaultProviderOverride: "openai",
+      defaultModelIdOverride: "gpt-4.1",
+    };
+
+    expect(resolveExecutorSessionModel("openai", "gpt-4.1", settings, assignedAgentRuntimeConfig)).toEqual({
+      provider: "mock",
+      modelId: "scripted",
+    });
+    expect(resolvePlanningSessionModel("openai", "gpt-4.1", settings, assignedAgentRuntimeConfig)).toEqual({
+      provider: "mock",
+      modelId: "scripted",
+    });
+    expect(resolveMergerSessionModel(settings, assignedAgentRuntimeConfig)).toEqual({
+      provider: "mock",
+      modelId: "scripted",
+    });
+    expect(resolveHeartbeatSessionModels(settings, assignedAgentRuntimeConfig)).toEqual({
+      defaultProvider: "mock",
+      defaultModelId: "scripted",
+      fallbackProvider: undefined,
+      fallbackModelId: undefined,
+    });
+  });
+
+  it("keeps existing behavior when test mode is inactive", () => {
+    const settings = {
+      executionProvider: "openai",
+      executionModelId: "gpt-4.1",
+      planningProvider: "openai",
+      planningModelId: "gpt-4.1",
+      defaultProviderOverride: "openai",
+      defaultModelIdOverride: "gpt-4.1",
+      defaultProvider: "openai",
+      defaultModelId: "gpt-4.1",
+    };
+
+    expect(resolveExecutorSessionModel("task-provider", "task-model", settings, assignedAgentRuntimeConfig)).toEqual({
+      provider: "anthropic",
+      modelId: "claude-sonnet-4-5",
+    });
+    expect(resolvePlanningSessionModel("task-provider", "task-model", settings, assignedAgentRuntimeConfig)).toEqual({
+      provider: "anthropic",
+      modelId: "claude-sonnet-4-5",
+    });
+    expect(resolveMergerSessionModel(settings, assignedAgentRuntimeConfig)).toEqual({
+      provider: "anthropic",
+      modelId: "claude-sonnet-4-5",
+    });
+    expect(resolveHeartbeatSessionModels(settings, assignedAgentRuntimeConfig)).toEqual({
+      defaultProvider: "anthropic",
+      defaultModelId: "claude-sonnet-4-5",
+      fallbackProvider: "openai",
+      fallbackModelId: "gpt-4.1",
+    });
+  });
+});