Create hub gateway example

[iannorriswork]

examples: create Hub::Gateway Ruby client for FDSH Hub Services

Problem

Integrated applications need a standardized and secure way to communicate with
the Hub Service API. Previously, there was no centralized Ruby implementation
that handled the specific requirements of the Hub environment, such as mTLS
authentication, TLS 1.2 enforcement, OAuth2 token management, and flexible
routing for local testing or environment-specific service paths.

Solution

Created examples/ruby-gateway.rb which implements Hub::Gateway, a comprehensive
Ruby client providing the following features:

• mTLS Support: Integrated certificate and key loading via OpenSSL,
  supporting HUB_CLIENT_CERT_PATH and HUB_CLIENT_KEY_PATH.
• Protocol Security: Enforced TLS 1.2 across all API calls to comply
  with security mandates.
• OAuth2 Flow: Automated client credentials grant flow for token
  acquisition and management.
• Network Flexibility: Implemented a resolve mechanism to override
  hostname resolution (similar to curl --resolve), enabling local mocking.
• Configurable Endpoints: Supported environment-specific service paths,
  allowing injection of custom URLs for services like Education Enrollment.
• Robust Error Handling: Standardized ApiError and AuthenticationError
  classes with explicit validation of SSL artifacts.

Result

This implementation serves as the foundational Ruby client for FDSH Hub
interactions. It simplifies the integration process for consumers by
abstracting connection complexity while ensuring compliance with technical
and security requirements.

Test Plan

Verified the gateway using a series of automated test scripts covering:

1. Authentication: Validated OAuth2 token fetching and error handling.
2. SSL/mTLS: Verified that certificates and keys load correctly and that
   the client fails fast with descriptive errors if files are missing.
3. Routing: Confirmed the resolve logic correctly maps hostnames to
   intended IPs for local tunnel testing.
4. Configuration: Demonstrated successful injection of custom service
   URLs and environment variables (HUB_*).
5. Functionality: Verified get_education_enrollment_v1 with a sample
   JSON payload provided in the documentation.

[neilmb]

Looks good. Is there somewhere in a Markdown doc that we should point people to look at these examples?