test(e2e-harness): delete broken test-helpers.spec.ts; document strategy (#499)

The spec imported `SendPromptAndWaitOptions` which was removed in PR #472
(harness helper consolidation), leaving a tsc error in the lint target
that has been silently failing the Library — lint gate. The chat-sidenav-
scrim no-output-native noise (resolved by #485) was masking it; subsequent
admin-merges let it through.

Two-part fix:

1. Delete the spec. Its original purpose ("locks in the type contract")
   was already covered by tsc itself; the deleted type meant the spec
   was just verifying a removed API.

2. Document the testing strategy in a header block on test-helpers.ts
   so future contributors know the absence of unit tests is intentional:
   - The 3 Playwright-dependent helpers are exercised end-to-end by the
     24 cockpit cap aimock e2es in the matrix.
   - `startAimock` (the only helper testable without a Page) has its own
     aimock-runner.spec.ts in the same directory.

Identified during the post-Task-#4 e2e audit (item #8).

Co-authored-by: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: blove

libs/e2e-harness/src/test-helpers.spec.ts

@@ -1,4 +1,20 @@
 // SPDX-License-Identifier: MIT
+//
+// Testing strategy for the helpers in this file:
+//
+// These helpers orchestrate Playwright `Page` APIs — `goto`, `getByRole`,
+// `locator`, `click`, `expect.toBeAttached/toBeVisible`. Unit-testing them
+// with a mock Page would mostly verify our wiring of Playwright's API, not
+// real behavior.
+//
+// Authoritative behavioral coverage lives in the cockpit cap aimock e2es
+// (24 specs across cockpit/{chat,langgraph,deep-agents}/*/angular/e2e/),
+// each of which exercises one of these helpers against a real Playwright
+// page driving the cap's UI through aimock-replayed responses. If any of
+// these helpers regress, the matrix lights up red across many caps.
+//
+// `aimock-runner.spec.ts` (in this same directory) covers `startAimock`
+// directly — it's the only helper testable without a real Page.
 import { expect, type Locator, type Page } from '@playwright/test';
 
 /**