Create a test suite for all integrations that inherit from `OpenAIChatGenerator`

[mpangrazzi]

Currently, we have 8 integrations (soon 9) that inherit from OpenAIChatGenerator on Haystack Core Integrations, each with nearly identical test files. So we have a lot of lines of duplicated test code.

Affected Integrations:

1. AIMLAPI - AIMLAPIChatGenerator
2. CometAPI - CometAPIChatGenerator (to be merged)
3. LlamaStack - LlamaStackChatGenerator
4. Meta Llama - MetaLlamaChatGenerator
5. Mistral - MistralChatGenerator
6. NVIDIA - NvidiaChatGenerator
7. OpenRouter - OpenRouterChatGenerator
8. STACKIT - STACKITChatGenerator
9. Together AI - TogetherAIChatGenerator

Each integration has nearly identical test files with only minor variations (model names, API keys, URLs).

With @sjrl we were discussing about creating an unified test suite which can reduce the code duplication between integrations.

[kudos07]

Thanks for raising this. I can look into drafting a shared test suite so each integration just specifies model-specific params (name, API URL, key env var).

Would you prefer this as a base ChatGeneratorTestMixin or a pytest parametrized approach?