refactor(showcase/harness): migrate pooled launchers to context checkout

Update all five pooled probe launchers for the context-pooled BrowserPool.
Each launcher's newContext() now checks out a pooled BrowserContext via
pool.acquire(opts) and the returned context-wrapper's close() releases it
via pool.release(ctx); the launcher-level close() is a no-op (no Browser is
held). The X-AIMock-Strict literal is dropped from every launcher (now
centralized in the pool); drivers still pass their per-probe X-AIMock-Context
/ X-Test-Id headers, which now flow through to pool.acquire. Driver run()
bodies are unchanged.

- d4 (createPooledE2eSmokeLauncher), e2e-parity
  (createPooledE2eParityLauncher): minimal transform.
- e2e-readiness (createPooledE2eDemosLauncher), d5
  (createPooledE2eDeepLauncher), d6 (createPooledE2eFullLauncher): the abort
  closure is re-targeted to close each open context (each releasing its
  pooled context) instead of force-releasing a held browser. d6's
  dead-browser re-acquire dance is removed, the pool only opens contexts on
  live browsers. Per-service Semaphore(FEATURE_CONCURRENCY/_D6) is preserved
  as the orthogonal per-service fan-out bound.

Driver tests reinterpret POOL_SIZE as maxContexts and assert per-context
acquire/release moves inUse by 1, abort closes open contexts with no browser
fork, and newContext(opts).extraHTTPHeaders forwards into pool.acquire.

Author: jpr5

showcase/harness/src/probes/drivers/d4-chat-roundtrip.ts

@@ -272,16 +272,17 @@ export function createPooledE2eSmokeLauncher(
   pool: BrowserPool,
 ): E2eBrowserLauncher {
   return async (): Promise<E2eBrowser> => {
-    const browser = await pool.acquire();
+    // CONTEXT-POOLED model: the launcher no longer acquires a Browser. Each
+    // `newContext()` checks out a pooled BrowserContext on a shared long-lived
+    // browser process (`pool.acquire`), and the wrapper's `close()` returns it
+    // (`pool.release`). The pool centralizes the X-AIMock-Strict default header;
+    // per-probe headers (X-AIMock-Context, X-Test-Id) flow through `ctxOpts`.
     return {
       async newContext(ctxOpts?: {
         extraHTTPHeaders?: Record<string, string>;
       }): Promise<E2eBrowserContext> {
-        const ctx = await browser.newContext({
-          extraHTTPHeaders: {
-            "X-AIMock-Strict": "true",
-            ...ctxOpts?.extraHTTPHeaders,
-          },
+        const ctx = await pool.acquire({
+          extraHTTPHeaders: ctxOpts?.extraHTTPHeaders,
         });
         return {
           async newPage(): Promise<E2ePage> {
@@ -296,12 +297,12 @@ export function createPooledE2eSmokeLauncher(
               close: () => page.close(),
             };
           },
-          close: () => ctx.close(),
+          close: () => pool.release(ctx),
         };
       },
-      close: async () => {
-        pool.release(browser);
-      },
+      // Launcher-level close is a no-op: contexts are released individually via
+      // each context-wrapper's close(). There is no Browser held to release.
+      close: async () => {},
     };
   };
 }