[tx-generator] Add --testnet-config-dir flag to json_highlevel

[palas]

Description

Adds a --testnet-config-dir flag to tx-generator json_highlevel that auto-discovers infrastructure config (socket path, signing key, node config, target nodes) from a cardano-testnet output directory, using the shared path conventions from Cardano.Node.Testnet.Paths.

Discovered infrastructure always overrides any user-provided values for those fields. All other config fields must be supplied by the user.

Closes #6513 (part 2 of #6510). Depends on #6518 (part 1).

What's new:

• Cardano.TxGenerator.Setup.TestnetDiscovery module — scans testnet dir for infrastructure config, merges with user config at JSON level (infrastructure always wins)
• json_highlevel parser now accepts either a full config file (existing behaviour preserved) or --testnet-config-dir with an optional partial config overlay
• Property-based test verifying infrastructure fields always override user-provided values, and that missing required non-infra fields cause failure

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:
  • property tests
  • roundtrip 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