feat: add CometAPI integration with CometAPIChatGenerator class and configuration

[garybadwal]

Related Issues

• fixes Feature request: Add CometAPI as an LLM provider #2269

Proposed Changes

Added a new CometAPIChatGenerator class that extends OpenAIChatGenerator to support the Comet API as a backend for chat generation.

• Configures api_base_url to point to https://api.cometapi.com/v1.
• Supports authentication via COMET_API_KEY environment variable (using Secret.from_env_var).
• Allows configuration of model name, streaming callbacks, tools, retry logic, and HTTP client kwargs, same as OpenAIChatGenerator.
• Default model is set to gpt-4o-mini.

This provides a seamless way to use CometAPI as a drop-in replacement for OpenAI within Haystack.

How did you test it?

• Verified initialization of CometAPIChatGenerator with and without explicit parameters.
• Tested that it correctly inherits all functionality from OpenAIChatGenerator.
• Manually checked API base URL override (api_base_url = https://api.cometapi.com/v1).
• Confirmed loading of API key from environment variable (COMET_API_KEY).

Notes for the reviewer

• The main change is introducing the CometAPIChatGenerator class.
• Implementation reuses all OpenAIChatGenerator logic, only customizing api_base_url and defaults.
• Please review whether we should add integration tests with a real Comet API key (currently avoided to keep CI clean).

Checklist

• I have read the [contributors guidelines](https://github.com/deepset-ai/haystack-core-integrations/blob/main/CONTRIBUTING.md) and the [code of conduct](https://github.com/deepset-ai/haystack-core-integrations/blob/main/CODE_OF_CONDUCT.md)
• I have updated the related issue with new insights and changes
• I added unit tests and updated the docstrings
• I've used one of the [conventional commit types](https://www.conventionalcommits.org/en/v1.0.0/) for my PR title (feat: in this case)

[garybadwal]

Hi @sjrl, just checking if we can merge this PR.

[mpangrazzi]

@garybadwal

I think you still need to address this?

Update: we should keep README consistent with other ones (so a lot more minimal).

The current README should be instead placed as a comet-api.md file in that folder, with links / examples.

[garybadwal]

@sjrl @mpangrazzi I have done the changes ✅

[garybadwal]

Hi @sjrl @mpangrazzi just wanted to check you both if you can give it a review, so I can fix things if required any changes.

[mpangrazzi]

@garybadwal I've checked the README but it's still not consistent with other integrations' README. It should have only package links and a Contributing section (example).

Then you can make another PR here with examples and an Usage section.

[garybadwal]

@mpangrazzi Done ✅, I made the changes i the Readme File.