Add `--testnet-config-dir` flag to `tx-generator json_highlevel`

[palas]

Context

Part 2 of #6510. Depends on #6517.

Adds a --testnet-config-dir flag to tx-generator json_highlevel that auto-discovers the 4 infrastructure config fields from a cardano-testnet output directory. Discovered infrastructure always overrides any user-provided values for those fields; all other config fields must be supplied by the user.

See #6510 for the full design.

Changes

New: bench/tx-generator/src/Cardano/TxGenerator/Setup/TestnetDiscovery.hs

Discovery:

• Scan testnet dir for node ports, socket path, signing key, config file (using Cardano.Node.Testnet.Paths)
• Localhost assumption documented in code (matches testnetDefaultIpv4Address in Testnet.Types)
• Discovered infrastructure (socket path, signing key, node config file, target nodes) always overrides user-provided values

Modify: bench/tx-generator/src/Cardano/Benchmarking/Command.hs

• New JsonHLSource ADT (FullConfig | TestnetSource) as alternative in parser
• Updated handler dispatching to discoverTestnetConfig

Modify: bench/tx-generator/tx-generator.cabal

• Add directory to build-depends
• Add Cardano.TxGenerator.Setup.TestnetDiscovery to exposed-modules
• Note: cardano-node is already a dependency (needed for Cardano.Node.Testnet.Paths)

New: tests

• Mock testnet directory test (generated programmatically from Cardano.Node.Testnet.Paths)
• Round-trip test (NixServiceOptions → JSON → NixServiceOptions)
• Property-based test verifying infrastructure fields always override user-provided values