refactor: optimize Cohere provider with response pooling and standardized request handling (#724)

## Summary

Refactored the Cohere provider to use a unified request handling approach and improved response object pooling for better performance and memory efficiency.

## Changes

- Implemented `completeRequest` method in Cohere provider to centralize API request handling
- Added object pooling for Cohere embedding responses to reduce memory allocations
- Standardized response handling across all Cohere API endpoints
- Added id, object, and model fields to Chat Completion responses from Bedrock and Cohere providers
- Updated list models functionality to use all available keys

## Type of change

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

## Affected areas

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

## How to test

Test the Cohere provider with various request types to ensure proper functionality:

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

# Test Cohere provider specifically
go test ./core/providers -run TestCohereProvider
```

## Breaking changes

- [x] No
- [ ] Yes

## Related issues

Improves performance and standardizes response handling across providers.

## Security considerations

No security implications as this is an internal refactoring of existing functionality.

## Checklist

- [x] I added/updated tests where appropriate
- [x] I verified builds succeed (Go and UI)
- [x] I verified the CI pipeline passes locally if applicable

Author: Pratham-Mishra04

core/changelog.md

@@ -1,4 +1,6 @@
 <!-- The pattern we follow here is to keep the changelog for the latest version -->
 <!-- Old changelogs are automatically attached to the GitHub releases -->
 
-- feat: Adds support for models list for providers
+- feat: Use all keys for list models request
+- refactor: Cohere provider to use completeRequest and response pooling for all requests
+- chore: Added id, object, and model fields to Chat Completion responses from Bedrock and Cohere providers