feat: add Requesty as a first-class OpenAI-compatible provider

[Thibaultjaigu]

What

Adds Requesty as a first-class, OpenAI-compatible LLM provider, mirroring the existing OpenRouter / Atlas Cloud wiring.

Requesty (https://requesty.ai) is an OpenAI-compatible LLM gateway. It uses the same provider/model naming as OpenRouter (e.g. openai/gpt-4o-mini) and exposes an OpenAI-shaped API at https://router.requesty.ai/v1.

Changes

• providers/catalog.py — register requesty in PROVIDER_DEFINITIONS (base URL https://router.requesty.ai/v1, OPENAI_CHAT transport, third_party_router risk label), and a requesty branch in classify_model_capabilities.
• providers/requesty.py — small helper module. Requesty's /v1/models capability fields differ from OpenRouter: it reports context_window (not context_length) and supports_tool_calling / supports_reasoning / supports_vision booleans (not a supported_parameters array), so these helpers map the real fields.
• providers/runtime.py — add requesty to list_configured_provider_ids and a dispatch branch in create_chat_model that builds a ChatOpenAICompatible client, mirroring the Atlas Cloud branch.
• providers/auth_store.py — map requesty → REQUESTY_API_KEY.
• models.py — base-URL constant, provider icon, and is_requesty_available().
• README.md — add Requesty to the two existing provider lists (one factual mention each).
• tests/test_requesty_first_class_provider.py — catalog registration, env-var mapping, runtime client construction (incl. missing-key error), and capability classification using Requesty's boolean fields.

Testing

• pytest tests/test_requesty_first_class_provider.py tests/test_provider_catalog.py → 51 passed (6 new + existing catalog suite, no regressions).
• Live check against the real endpoint: POST https://router.requesty.ai/v1/chat/completions with openai/gpt-4o-mini → HTTP 200 with a valid completion.

The key is read from the OS keyring via the existing get_provider_secret path, exactly like the other providers; no new env-file surface is introduced.

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I work at Requesty. This mirrors the existing OpenRouter provider as closely as possible. Happy to adjust wording/placement or close it if it's not a fit.