[FEATURE] Add Requesty as an OpenAI-compatible LLM provider

Feature Area

Integration with external tools

Is your feature request related to a an existing bug? Please link it here.

NA

Describe the solution you'd like

Add Requesty as a supported provider in the OpenAI-compatible provider set, next to OpenRouter. Requesty is an OpenAI-compatible LLM router (https://router.requesty.ai/v1) that uses the same provider/model naming convention as OpenRouter, so a model string like requesty/openai/gpt-4o-mini should route through the existing openai_compatible completion path with REQUESTY_API_KEY for auth and an optional REQUESTY_BASE_URL override (for the EU endpoint https://router.eu.requesty.ai/v1).

Concretely that means a ProviderConfig entry for requesty in llms/providers/openai_compatible/completion.py, the requesty prefix registered in llm.py (supported providers, prefix map, pattern-match branch, openai_compatible set), tests mirroring the OpenRouter cases, and a short docs section.

Describe alternatives you've considered

Today users can already reach Requesty by pointing LLM(model="openai/...", base_url="https://router.requesty.ai/v1", api_key=...) at the router by hand. That works but loses the provider/model convenience, needs the base URL repeated in every config, and is not discoverable from the docs. A first-class prefix matches how OpenRouter, DeepSeek and Cerebras are already handled.

Additional context

There is an implementation ready in #6273 (tests pass, ruff clean, live call against the router verified). The PR was auto-closed because first-time contributors need an associated issue, so this is that issue.

Disclosure: I work at Requesty. Happy to adjust anything to match project conventions.

Willingness to Contribute

Yes, I'd be happy to submit a pull request

Start in llms/providers/openai_compatible/completion.py and llm.py, then review the existing OpenRouter provider cases and the related documentation section. Done means Requesty is configured with its prefix, authentication and optional base URL, tests mirror the OpenRouter coverage, and the provider is documented.