Part of A4A-2379

Proposed Changes

• Add a pilot allowlist for the a4a-partner-directory-lead-matching visibility gate.
• Keep the global config feature flag as the top-level override.
• Use the same agency-scoped visibility check for the partner-directory menu item and route guard.
• Add focused test coverage for the helper.

Why are these changes being made?

• Lead matching should stay disabled by default in shared environments while selected pilot
  agencies get access to the feature.
• The existing implementation only supports a global flag, which is too broad for the pilot
  rollout.

Testing Instructions

• Run:
  yarn test-client client/a8c-for-agencies/sections/partner-directory/lib/test/lead-matching-visibility.test.ts --runInBand
• With the global flag disabled, verify agencies 232667176, 251102500, 234036126,
  234278359, and 232640028 can access /partner-directory/lead-matching and see the menu
  entry.
• Verify a non-pilot agency still does not see the menu item and is redirected away from the
  lead-matching route.

Pre-merge Checklist

• Has the general commit checklist been followed? (PCYsg-hS-p2)
• Have you written new tests for your changes?
• Have you tested the feature in Simple (P9HQHe-k8-p2), Atomic (P9HQHe-jW-p2), and self-hosted Jetpack sites (PCYsg-g6b-p2)?
• Have you checked for TypeScript, React or other console errors?
• Have you tested accessibility for your changes? Ensure the feature remains usable with various user agents (e.g., browsers), interfaces (e.g., keyboard navigation), and assistive technologies (e.g., screen readers) (PCYsg-S3g-p2).
• Have you used memoizing on expensive computations? More info in Memoizing with create-selector and Using memoizing selectors and Our Approach to Data
• Have we added the "[Status] String Freeze" label as soon as any new strings were ready for translation (p4TIVU-5Jq-p2)?
  • For UI changes, have we tested the change in various languages (for example, ES, PT, FR, or DE)? The length of text and words vary significantly between languages.
• For changes affecting Jetpack: Have we added the "[Status] Needs Privacy Updates" label if this pull request changes what data or activity we track or use (p4TIVU-aUh-p2)?