fix(tui): scroll the /model Roles view so clipped rows stay reachable

The Roles panel renderer looped from the first row with a hard height cap and no scroll offset, so roles and model-keyed fallback chains past the visible height were never drawn and unreachable by keyboard, with no truncation indicator. Unlike the sibling provider list (model-browser windows via #windowStart/#ensureSelectedVisible), the Roles panel had no equivalent.

Window #renderRolesView around #roleIndex via #ensureRoleVisible, offset mouse hit-testing by the scroll start bounded to the visible count, and draw an up/down '+N more' hint when the list is clipped.

Fixes #8817

packages/coding-agent/test/model-hub.test.ts

@@ -92,12 +92,13 @@ function createHub(options: {
 	registry?: RegistryOverrides;
 	hub?: ModelHubOptions;
 	callbacks?: Partial<ModelHubCallbacks>;
+	terminalRows?: number;
 }): HubHarness {
 	installTestTheme();
 	const modelsFn = typeof options.models === "function" ? options.models : () => options.models as Model[];
 	const settings = options.settings ?? Settings.isolated({});
 	const registry = makeRegistry(modelsFn, options.registry);
-	const ui = { requestRender: vi.fn(), terminal: { rows: 40 } } as unknown as TUI;
+	const ui = { requestRender: vi.fn(), terminal: { rows: options.terminalRows ?? 40 } } as unknown as TUI;
 	const onAssign = vi.fn();
 	const onUnassign = vi.fn();
 	const onLoginRequest = vi.fn();
@@ -709,6 +710,35 @@ describe("ModelHub", () => {
 			expect(onFallbackChainChange).toHaveBeenLastCalledWith("default", ["test/model-b"]);
 		});
 
+		test("windows the roles list so model-keyed chains past the panel height stay reachable", () => {
+			const settings = Settings.isolated({
+				// Model-keyed chains sort alphabetically; the unique tail key lands last.
+				"retry.fallbackChains": {
+					"aa-provider/head-chain": ["x/y"],
+					"mm-provider/mid-chain": ["x/y"],
+					"zz-provider/tail-chain-marker": ["x/y"],
+				},
+			});
+			// A short terminal makes the built-in roles alone fill the panel, so the
+			// model-keyed chains that follow the separator land below the fold. The
+			// chain keys are not available models, so no role auto-assignment leaks
+			// their names into the visible role rows.
+			const { hub } = createHub({ models: [makeModel("test", "solo")], settings, terminalRows: 16 });
+
+			enterRolesView(hub);
+			const top = normalize(hub.render(120));
+			// The alphabetically last model-keyed chain is clipped, but the panel
+			// now advertises the hidden rows instead of dropping them silently.
+			expect(top).not.toContain("tail-chain-marker");
+			expect(top).toContain("more");
+
+			// Wrapping up from the top row lands on the trailing "+ New fallback…"
+			// row; the window scrolls to the bottom and reveals the clipped chain.
+			hub.handleInput(UP);
+			const bottom = normalize(hub.render(120));
+			expect(bottom).toContain("tail-chain-marker");
+		});
+
 		test("clicking a roles row hits the row under the pointer", () => {
 			const a = makeModel("test", "model-a");
 			const { hub } = createHub({ models: [a], scoped: true });