Add Telnyx AI services integration example

[a692570]

Summary

This PR adds documentation and configuration examples for integrating EchoKit Server with Telnyx AI services.

Why Telnyx?

Telnyx provides a compelling option for EchoKit users:

• OpenAI-Compatible API: All endpoints follow OpenAI specifications, enabling drop-in compatibility with EchoKit's existing architecture
• 53+ AI Models: Access to GPT-4, Claude, Llama, Mistral, and open-source models through a single API
• Global Edge Network: Low-latency inference from data centers worldwide
• Unified Billing: Single API key for ASR, TTS, and LLM services
• Competitive Pricing: Pay-per-use with transparent, per-token pricing

Changes

• examples/telnyx/config.toml: Ready-to-use configuration with environment variable support
• examples/telnyx/README.md: Complete integration guide with setup instructions, configuration reference, and troubleshooting

Testing

The configuration follows the existing config.toml structure and uses the same OpenAI-compatible endpoints that EchoKit already supports. Users can test by:

1. Setting TELNYX_API_KEY environment variable
2. Copying the example config
3. Running EchoKit Server

Documentation

The README includes:

• Quick start guide
• Configuration reference for ASR, TTS, and LLM
• Available model options
• Troubleshooting tips
• Links to Telnyx documentation