cardano-testnet: Recompute genesis hashes in configuration.yaml if missing

[palas]

Problem

create-env deliberately writes configuration.yaml without genesis hashes (WithoutHashes), so users can edit genesis files without recomputing hashes. However, cardanoTestnet (the runtime phase) never rewrites configuration.yaml, so when using cardano --node-env, the config lacks hashes entirely.

In the direct cardano path, createTestnetEnv uses WithHashes, so the config always has hashes. This is an inconsistency between the two paths.

Proposed fix

At the start of cardanoTestnet in Testnet/Start/Cardano.hs (~L241), after reading the config file, check if ByronGenesisHash key is present. If not, call createConfigJson and rewrite configuration.yaml. This is ~15 lines of additive code. If hashes are already present, nothing changes.

Depends on

None (independent)

Type: Consistency fix

Effort: Low (~15 lines)

Risk: Low

[palas]

This one may be a bit unnecessary, and I am not sure whether we want to preserve the principle that, in principle, we do not touch the config files. This principle would affect #6455 as well. So I'll postpone