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

[palas]

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

• cardano-testnet: Add TestnetEnvManifest to systematise cross-phase data passing #6461 (TestnetEnvManifest must exist first)

Type: Refactoring

Effort: Low (~10 lines)

Risk: Low (clear error on mismatch; no silent breakage)