feat: add Modelsell chat model provider

[modelsell]

What changed

Adds Modelsell as a named Flowise chat model provider using the existing OpenAI-compatible ChatOpenAI implementation.

• adds a dedicated Modelsell API credential with a link to the API key page
• discovers available model IDs dynamically from GET https://modelsell.com/v1/models
• sends chat requests through the fixed https://modelsell.com/v1 base URL
• supports streaming, sampling controls, timeout, caching, and additional headers
• includes focused tests for model discovery, credential validation, and chat-model configuration

Why

Modelsell already works through a manually configured OpenAI-compatible node, but a named integration makes the provider discoverable and avoids repeated endpoint and model-ID configuration. Dynamic discovery also avoids maintaining a hardcoded model catalog.

Closes #6675.

Validation

• pnpm --filter flowise-components test -- ChatModelsell.test.ts --runInBand (3 passed)
• targeted ESLint for the new credential, node, and test files
• git diff --check

The full component lint was also attempted and currently reports nine pre-existing unused eslint-disable directives outside these files. The full component build reaches TypeScript compilation but is blocked in this local workspace by missing optional storage dependencies such as multer, multer-s3, and turndown.

Disclosure: I represent Modelsell.

[gemini-code-assist]

Caution

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