cardano-testnet: Enforce SPOs come first and split node list into SPO and relay

[palas]

Description

The NodeOption type was refactored from a sum type (SpoNodeOptions | RelayNodeOptions) into a record, and a new TestnetNodeOptions container enforces at the type level that there is at least one SPO (NonEmpty) and that SPOs come before relays (by structure). The parser also validates this ordering.

readNodeOptionsFromEnv now validates that node directories are consecutively numbered and that SPOs come before relays, matching the assumptions made by cardanoTestnet.

Follow-up PR: #6559 (adds --nodes flag for per-node binary configuration on top of this)

I would recommend looking at the diff with white-space disabled.

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