[BUG]: resolve api_key from CHROMA_API_KEY env var

[tazarov]

Closes #6104

Description of changes

CloudClient was not using the CHROMA_API_KEY environment variable when the api_key parameter was omitted. The resolved value was stored in arg.value but never reassigned to the api_key variable.

• Improvements & Bug fixes
  • Changed required_args from list to dict for name-based access, and updated api_key assignment to use the resolved value. Added tests for env var resolution and param precedence.

Test plan

• Tests pass locally with pytest for python

Migration plan

N/A

Observability plan

N/A

Documentation Changes

N/A

[github-actions]

Reviewer Checklist

Please leverage this checklist to ensure your code review is thorough before approving

Testing, Bugs, Errors, Logs, Documentation

• Can you think of any use case in which the code does not behave as intended? Have they been tested?
• Can you think of any inputs or external events that could break the code? Is user input validated and safe? Have they been tested?
• If appropriate, are there adequate property based tests?
• If appropriate, are there adequate unit tests?
• Should any logging, debugging, tracing information be added or removed?
• Are error messages user-friendly?
• Have all documentation changes needed been made?
• Have all non-obvious changes been commented?

System Compatibility

• Are there any potential impacts on other parts of the system or backward compatibility?
• Does this change intersect with any items on our roadmap, and if so, is there a plan for fitting them together?

Quality

• Is this code of a unexpectedly high quality (Readability, Modularity, Intuitiveness)