fix(CDPBrowser): deterministic Obscura suite — await within-block promise chains in shared webapi tests; null-safe assertion message templating

Flake 1 ("should respect form fields inside within block") and Flake 2 (the
"kills a SIGTERM-ignoring child within bounds" phantom) were one bug, not two.

test/helper/webapi.js's `within tests` fired `I._withinBegin(locator).then()...`
without awaiting/returning it, racing the test's own later `await`ed
assertions against the still-in-flight async scope setup. Under load this
produced a real AssertionFailedError with a `null` param. lib/utils.js's
template() only guarded `undefined`, so formatting that error inside Mocha's
own epilogue threw `null.toString()`, crashing the reporter mid-print; Mocha's
uncaughtException recovery then misattributed the crash to whatever test was
`currentRunnable` (the last-run spawn-lifecycle test), producing a blank,
EXIT:0 "phantom" failure on an otherwise fully deterministic test (verified
15/15 clean in isolation).

Fixes:
- test/helper/webapi.js: await the three unawaited within-block promise
  chains that precede further awaited assertions in the same test body.
- lib/utils.js: template() now treats `null` the same as `undefined`
  (renders as empty string instead of throwing) — a deliberate divergence
  from 3.x, which has the identical latent bug.

Verified via 3 consecutive full Obscura suite runs (179 passing/122
pending/0 failing, exit 0 each time, instrumented to confirm zero unhandled
rejections escalate to a Mocha failure), isolated within-describe passes on
Obscura/Chrome/Puppeteer/Playwright, and a clean Chrome regression run
(201/112/0, unchanged). Full root-cause writeup in
.superpowers/flake-report.md.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

Author: DavertMik

lib/utils.js

@@ -108,7 +108,7 @@ export const methodsOfObject = function (obj, className) {
 export const template = function (template, data) {
   return template.replace(/{{([^{}]*)}}/g, (a, b) => {
     const r = data[b]
-    if (r === undefined) return ''
+    if (r === undefined || r === null) return ''
     return r.toString()
   })
 }

test/helper/webapi.js

@@ -1620,7 +1620,7 @@ export function tests() {
       if (I.capabilities?.layout === 'none') this.skip() // visibility requires a real layout engine
       await I.amOnPage('/form/example4')
       await I.seeElement('#navbar-collapse-menu')
-      I._withinBegin('#register')
+      await I._withinBegin('#register')
         .then(() => I.see('E-Mail'))
         .then(() => I.dontSee('Toggle navigation'))
         .then(() => I.dontSeeElement('#navbar-collapse-menu'))
@@ -1638,7 +1638,7 @@ export function tests() {
       await I.seeInField('Hasło', '12345')
       await I.checkOption('terms')
       await I.seeCheckboxIsChecked('terms')
-      I._withinBegin({ css: '.form-group' })
+      await I._withinBegin({ css: '.form-group' })
         .then(() => I.see('E-Mail'))
         .then(() => I.dontSee('Hasło'))
         .then(() => I.dontSeeElement('#navbar-collapse-menu'))
@@ -1661,7 +1661,7 @@ export function tests() {
     it('should execute within block 2', async () => {
       await I.amOnPage('/form/example4')
       await I.fillField('Hasło', '12345')
-      I._withinBegin({ xpath: '//div[@class="form-group"][2]' })
+      await I._withinBegin({ xpath: '//div[@class="form-group"][2]' })
         .then(() => I.dontSee('E-Mail'))
         .then(() => I.see('Hasło'))
         .then(() => I.grabTextFrom('label'))