Copilot CLI BYOK article lacks a third-party gateway example

[harryvgiunta]

Issue body

The Copilot CLI BYOK article (content/copilot/how-tos/copilot-cli/customize-copilot/use-byok-models.md) documents COPILOT_PROVIDER_BASE_URL / COPILOT_PROVIDER_API_KEY / COPILOT_MODEL with OpenAI and local-Ollama examples only.

The openai provider type explicitly covers "any other OpenAI Chat Completions API-compatible endpoint", but a reader connecting a third-party gateway has no worked example showing:

• the three variables with gateway-style values,
• which gateway models satisfy the tool calling + streaming requirements,
• how a provider-enforced plan context window interacts with COPILOT_PROVIDER_MAX_PROMPT_TOKENS.

Proposal: add a short third-party gateway example block to "Connecting to an OpenAI-compatible endpoint". A concrete candidate is Yolo-Auto (https://yolo-auto.com/v1), whose yolo and yolo-small models support tool calling, streaming, and image inputs, and which enforces a 131072-token context window per plan.

[subatoi]

Hi, I'd suggest opening a community discussion about this proposal, but we'd only accept content relating to a specific product under very specific circumstances, in any case. As such I'm going to close this and the linked PR out.