Add Requesty provider to the CLI

[Thibaultjaigu]

Adds Requesty (https://requesty.ai) as a provider option to the CLI (src/Cli), mirroring the existing Provider.OpenRouter.

• src/Cli/src/Models/Provider.cs: new Requesty enum value.
• src/Cli/src/Helpers.cs: base-URL switch arm (https://router.requesty.ai/v1) and API-key switch arm (REQUESTY_API_KEY).

Requesty is OpenAI-compatible (Authorization: Bearer, provider/model naming), so it works through the existing OpenAI client path. --provider requesty resolves via the existing case-insensitive enum parsing (Option<Provider>), so no extra string mapping was needed. The core SDK and the integration tests are untouched. API keys: https://app.requesty.ai/api-keys

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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