[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.

Two orthogonal merge flags (--fill-defaults/--no-fill-defaults, --force-infra/--no-force-infra) control how discovered config is merged with user-provided JSON — see #6510 for the full design.

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

What's new:

• Cardano.TxGenerator.Setup.TestnetDiscovery module — scans testnet dir, detects era from configuration.yaml via npcTestStartingEra, merges with user config at JSON level
• 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 with randomised user configs and all flag combinations, covering merge correctness for infra, required, and optional fields

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