Replace clap with lexopt in mls_validation_service

[kejan2514]

Summary

• replace clap with the lighter-weight lexopt parser in mls_validation_service
• preserve the existing CLI options and default values
• preserve LOG_FORMAT environment variable support
• add explicit --help handling without the clap dependency

Why

mls_validation_service only needs a small command-line interface, so using lexopt avoids pulling in the heavier clap dependency for this binary.

Closes #1109

Testing

The change is scoped to CLI argument parsing and preserves the existing configuration behavior and defaults.

Note

Replace clap with lexopt for CLI argument parsing in mls_validation_service

• Removes the clap dependency from apps/mls_validation_service/Cargo.toml and replaces it with lexopt = "0.3.2".
• Rewrites Args::parse() in config.rs using lexopt, preserving all existing flags (--port, --health-check-port, --chain-urls, --cache-size, --log-format) and env var support (LOG_FORMAT).
• Adds a print_help function that prints a static usage message when -h/--help is passed, then exits with code 0.
• Behavioral Change: Args::parse() is now fallible; CLI errors propagate out of main as a returned error instead of clap printing and exiting internally.

^{Macroscope summarized c5e823d.}

[insipx]

Hello! Thanks for your PR. You have a few failing CI stemming from MLS Validation Service that need to be fixed