Add Requesty as a provider

[Thibaultjaigu]

Adds Requesty as a provider, mirroring the existing OpenRouter provider end-to-end.

Requesty (https://requesty.ai) is an OpenAI-compatible LLM router — base URL https://router.requesty.ai/v1, provider/model naming, Authorization: Bearer auth. It reuses the same OpenAI-compatible chat-client path OpenRouter uses.

Changes:

• Models/Providers/Provider.cs: Requesty enum member.
• Models/Providers/Requesty/RequestyConfiguration.cs (new): IProviderConfiguration mirroring OpenRouterConfiguration (BaseAddress = https://router.requesty.ai/v1, DefaultModel = openai/gpt-4o-mini).
• Models/ServiceCollectionExtensions.cs: AddRequestyChatClient() via the OpenAI-compatible client (mirrors OpenRouter).
• Users/Entitlement.cs + Users/Models/Entitlements.cs: EnableRequestyProvider.
• AddIn/CellmAddIn.cs, AddIn/UserInterface/Ribbon/RibbonModelGroup.cs: config binding, HTTP client, provider list, ribbon base-address case.
• appsettings.json: RequestyConfiguration section (verified model ids).
• Cellm.csproj: embeds Requesty.svg.
• AddIn/UserInterface/Resources/Requesty.svg (new): placeholder icon (neutral glyph, not a Requesty logo) — happy to swap for the official mark.
• Tests: added Requesty to the unit pipeline (mock client) and the integration fixture helpers; left the live-key integration Theories untouched (Requesty isn't in CI secrets, matching how OpenRouter is gated).

Note: I couldn't run dotnet build locally (net9.0-windows / WinForms target on a non-Windows machine), so this hasn't been compiled here — please give it a build. Every reference was mirrored 1:1 against OpenRouter.

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

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