docs(credit-usage-api): add new endpoint documentation for credit usage

[RutamBhagat]

Fixes: #1000

Add credit usage API docs

This PR adds docs for the /credit-usage endpoint to the v1-openapi.json file. The docs includes:

• Endpoint definition with GET method.
• Bearer authentication requirement.
• Success response schema with remaining credits.
• Error responses for 404 and 500 status codes.

This change makes sure that the API docs reflect the functionality of the credit usage endpoint

[RutamBhagat]

Hey @nickscamara ,

I've updated v1-openapi.json to include the credit usage endpoint, specifically documenting the GET /credit-usage path, its response schema, and error handling.

Please confirm if these OpenAPI changes are sufficient or if you'd like me to update anyy other documentation files to reflect this new endpoint.

The GitHub issue was a bit ambiguous, as it only included the title and no further description.