cardano-testnet: Split testnet options into purpose-specific types

[palas]

Description

Split the monolithic CardanoTestnetOptions record into purpose-specific types so that:

1. --node-env and --num-pool-nodes are structurally mutually exclusive in the CLI parser (via <|>, not optional fields that silently coexist).
2. No dummy/unused values are passed between call sites — each function receives only the fields it actually uses.
3. The type structure matches the three real execution modes: create-only (create-env), create-and-run (cardano), and run-from-existing-env (cardano --node-env).

New type structure

• TestnetCreationOptions — everything needed to create a testnet environment: nodes, era, max supply, num DReps, genesis options, on-chain params. Has a Default instance for tests.
• TestnetRuntimeOptions — everything needed to run nodes: epoch state logging, gRPC, KES source. Has a Default instance for tests.
• TestnetEnvOptions — --node-env path + timestamp update policy.
• CardanoTestnetCliOptions — sum type: StartFromScratch StartFromScratchOptions | StartFromEnv StartFromEnvOptions, making the two modes exclusive at the type level.

Checklist

• Commit sequence broadly makes sense and commits have useful messages
• New tests are added if needed and existing tests are updated. These may include:
  • golden tests
  • property tests
  • roundtrip tests
  • integration tests
    See Running tests for more details
• Any changes are noted in the CHANGELOG.md for affected package
• The version bounds in .cabal files are updated
• CI passes. See note on CI. The following CI checks are required:
  • Code is linted with hlint. See .github/workflows/check-hlint.yml to get the hlint version
  • Code is formatted with stylish-haskell. See .github/workflows/stylish-haskell.yml to get the stylish-haskell version
  • Code builds on Linux, MacOS and Windows for ghc-9.6 and ghc-9.12
• Self-reviewed the diff