Remove format-sensitive load shedder registration checks

[skovranek]

Changes

1. Remove exact source-string matching for health-route and load-shedder registration from scripts/availability-policy.mjs.
2. Allow equivalent formatting and middleware variable names without failing the load-shedding probe.
3. Remove the obsolete healthRegisteredBeforeLoadShedder output field. The companion lesson update checks health bypass against the running app.
4. Retain the concurrency probe for rejecting excess work, returning Retry-After, and releasing capacity after completion or disconnection.
5. Add redirect: "manual" to the timeout probe so it inspects the checkout response instead of following the fake PawPal redirect.

Verification

6. Pass npm run lint and npm run typecheck.
7. Run all 18 chapter 11 before/after cases with matching copies of this script: all nine preceding snapshots fail, and all nine completed solutions pass.
8. Verify that the timeout before snapshot produces 302 and one created order, then fails the expected assertion without a DNS error. The completed solution produces 503, creates no orders, and passes.
9. Verify that formatting variants pass and missing registration, incorrect health placement, and header-only middleware are rejected.
10. Confirm the starter script matches all 74 course complete-directory copies.

Companion PR

11. Course changes: https://github.com/bootdotdev/go-api-gate/pull/10010. Publish that lesson update before merging this starter change so the CLI no longer expects the removed JSON field.