cardano-testnet: Ignore CLI --num-pool-nodes when TestnetEnvManifest is present

## Problem In the two-step flow (`create-env` then `cardano --node-env`), the user can pass different `--num-pool-nodes` values to each invocation. If step 1 creates 5 pool nodes but step 2 is told to expect 3, `cardanoTestnet` will look for key files that do not exist or miss topology entries, causing silent failures. ## Proposed fix Once `TestnetEnvManifest` exists (#6461), have `cardanoTestnet` read the manifest when a pre-existing env is provided and use `manifestNodes` to determine node count and types instead of the CLI flags. - If CLI `--num-pool-nodes` is specified and differs from the manifest, emit a warning (or error). - ~10 lines in `Parsers/Run.hs` (read manifest) and `Testnet/Start/Cardano.hs` (use manifest nodes). ## Depends on - #6461 (TestnetEnvManifest must exist first) ## Type: Refactoring ## Effort: Low (~10 lines) ## Risk: Low (clear error on mismatch; no silent breakage)