[test-recorder] --backend accepts semantically invalid URLs; add validation and a bounded health probe

### Problem
`--backend` URL validation is syntax-only (`src/devserver/distributions.ts:56-83`, `src/devserver/manager.ts:48-50`). URLs containing embedded credentials, query strings, fragments, arbitrary paths, or unreachable hosts pass validation and get injected into the dev-server environment, failing later with confusing symptoms.

### Suggested fix
Reject credentials/query/fragment components, document whether path prefixes are supported, and run a bounded (2-3s) health/identity probe against the backend before starting the dev server — with non-dev-friendly error copy on failure.

_Line references are against branch `cb/comfy-test-usability` (PR #15837); adjust after merge._