Add comprehensive test coverage for all request builder helper functions
in helpers_test.go. The new test file verifies:
1. CreateProxyRequest() - correct method, URL, headers, and body handling
2. CreateMessagesRequest() - POST /v1/messages requests
3. CreateStreamingRequestBody() - valid streaming JSON with stream:true
4. CreateNonStreamingRequestBody() - valid non-streaming JSON
5. CreateStreamingMessagesRequest() - streaming requests
6. CreateNonStreamingMessagesRequest() - non-streaming requests
7. CreateTestRequestBody() - custom request bodies with model/messages/stream
8. MustCreateTestRequestBody() - panic behavior on error
All 33 sub-tests pass, confirming that the request builder functions
create valid HTTP requests with correct structure, headers, and bodies
for both streaming and non-streaming scenarios.
Co-Authored-By: Claude <noreply@anthropic.com>
