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.

[CLAassistant]

Thank you for your submission! We really appreciate it. Like many open source projects, we ask that you all sign our Contributor License Agreement before we can accept your contribution.
1 out of 2 committers have signed the CLA.

✅ HavenDV
❌ Thibaultjaigu
_{You have signed the CLA already but the status is still pending? Let us recheck it.}

[HavenDV]

Thank you, Thibault — Requesty is a sensible fit for the existing OpenAI-compatible CLI path, and the original CI run passed. The only current blocker is the pending CLA check. Could you please sign it when convenient? Once that is green, we can update your branch to current main, add focused provider-resolution coverage if needed, and merge without squashing so your original commit remains in history.