feat: add auto-fixture for Istanbul coverage collection

Add a `_coverageCollector` auto-fixture that collects
`window.__coverage__` from the browser after each test and writes
it to a JSON file. This enables E2E coverage collection for
instrumented dynamic plugins without modifying any test files.

Guarded by E2E_COLLECT_COVERAGE=1 — zero overhead when disabled.
Coverage files are written to coverage/istanbul/ (or
COVERAGE_OUTPUT_DIR) and consumed by the overlay repo's coverage
reporter for merging into lcov and uploading to Codecov.

Part of the E2E coverage pipeline: RHIDP-13411

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: gustavolira

src/playwright/fixtures/test.ts

@@ -3,12 +3,14 @@ import { test as base } from "@playwright/test";
 import { LoginHelper, UIhelper } from "../helpers/index.js";
 import { runOnce } from "../run-once.js";
 import { $ } from "../../utils/bash.js";
+import fs from "node:fs";
 import path from "path";
 
 type RHDHDeploymentTestFixtures = {
   rhdh: RHDHDeployment;
   uiHelper: UIhelper;
   loginHelper: LoginHelper;
+  _coverageCollector: void;
 };
 
 type RHDHDeploymentWorkerFixtures = {
@@ -61,6 +63,35 @@ const baseTest = base.extend<
     },
     { scope: "test" },
   ] as const,
+  _coverageCollector: [
+    async ({ page }, use, testInfo) => {
+      await use();
+      if (process.env.E2E_COLLECT_COVERAGE !== "1") return;
+      try {
+        const coverage = await page.evaluate(
+          () =>
+            (
+              globalThis as unknown as {
+                __coverage__?: Record<string, unknown>;
+              }
+            ).__coverage__,
+        );
+        if (!coverage) return;
+        const dir = path.resolve(
+          process.cwd(),
+          process.env.COVERAGE_OUTPUT_DIR || "coverage/istanbul",
+        );
+        fs.mkdirSync(dir, { recursive: true });
+        fs.writeFileSync(
+          path.join(dir, `${testInfo.testId}-${Date.now()}.json`),
+          JSON.stringify(coverage),
+        );
+      } catch {
+        // Best-effort: page may have crashed or been closed
+      }
+    },
+    { auto: true, scope: "test" },
+  ],
 });
 
 export const test = Object.assign(baseTest, {