feat: added responses stream in cohere (#608)

## Summary

Implement Cohere ResponsesStream support and refactor token usage handling to improve reliability and reduce code duplication.

## Changes

- Added ResponsesStream implementation for Cohere provider
- Refactored token usage handling to use non-pointer float64 values
- Fixed content and tool call handling in streaming responses
- Simplified delta message content handling in stream events
- Reorganized code structure for better readability and maintainability

## Type of change

- [x] Bug fix
- [x] Feature
- [x] Refactor
- [ ] Documentation
- [ ] Chore/CI

## Affected areas

- [x] Core (Go)
- [ ] Transports (HTTP)
- [x] Providers/Integrations
- [ ] Plugins
- [ ] UI (Next.js)
- [ ] Docs

## How to test

Test the Cohere provider with ResponsesStream functionality:

```sh
# Core/Transports
go version
go test ./...

# Test ResponsesStream with Cohere
curl -X POST "http://localhost:8000/v1/providers/cohere/responses-stream" \
  -H "Content-Type: application/json" \
  -H "Authorization: Bearer YOUR_API_KEY" \
  -d '{
    "model": "command",
    "messages": [
      {"role": "user", "content": "Tell me about quantum computing"}
    ]
  }'
```

## Screenshots/Recordings

N/A

## Breaking changes

- [ ] Yes
- [x] No

## Related issues

Implements ResponsesStream support for Cohere provider

## Security considerations

No security implications.

## Checklist

- [x] I read `docs/contributing/README.md` and followed the guidelines
- [x] I added/updated tests where appropriate
- [x] I updated documentation where needed
- [x] I verified builds succeed (Go and UI)
- [x] I verified the CI pipeline passes locally if applicable

Author: akshaydeo