Add --quiet and --json output modes to the validate command #30

Goal

Give openapi-to-rust validate the same automation-friendly output controls as generate and init.

Scope

Add --quiet and --json to the validate subcommand in src/bin/openapi-to-rust.rs. Reuse the existing machine-readable success/error conventions; do not create a second configuration-validation path.

Acceptance criteria

• validate --quiet emits no output on success.
• validate --json emits exactly one JSON value on stdout for success and failure, with no human text on stderr.
• Invalid configuration still exits unsuccessfully.
• --quiet and --json conflict consistently with the other commands.
• End-to-end coverage is added to tests/cli_workflow_test.rs.
• README CLI examples mention the new modes.

This should remain a focused CLI/output change with no generated-code changes.