Complete Test Coverage for OpenAI Completion Provider

[Furisto]

Overview

The OpenAI completion provider (`openai_completion.go`) is fully implemented with streaming, function calling, and token usage tracking. However, its test file (`openai_completion_test.go`) is empty with only a stub test.

Current State

• ✅ Provider implementation complete with streaming support
• ✅ Function/tool calling fully integrated
• ✅ Token usage tracking with cache metrics
• ✅ Proper error handling and structured logging
• ❌ Test file only contains an empty stub test

What Needs to be Done

Implement comprehensive unit tests for the OpenAI completion provider to ensure reliability and catch regressions.

Test Coverage Requirements

1. Basic Invocation Tests

   • Test successful model invocation with valid inputs
   • Verify correct message formatting and transformation
   • Validate response structure and content

2. Streaming Tests

   • Test streaming callback functionality
   • Verify chunks are properly accumulated
   • Test handling of empty/incomplete streams

3. Function Calling Tests

   • Test tool transformation to OpenAI format
   • Verify tool calling with `EnableFunctionCalling` enabled
   • Test handling of multiple tool calls
   • Test parallel tool call execution

4. Error Handling Tests

   • Missing or empty API key
   • Missing required fields (model, system prompt, messages)
   • Invalid message types
   • API errors and network failures
   • Invalid model profiles

5. Message Transformation Tests

   • User messages with text content
   • Model messages with text and tool calls
   • Tool result messages
   • Mixed message types in conversation history

6. Token Usage Tests

   • Verify input/output token counts are tracked
   • Test cache read token metrics
   • Validate cache hit ratio calculation

7. Edge Cases

   • Empty message list
   • Very long messages
   • Special characters and unicode
   • Large number of tools

Files to Modify

• `backend/model/openai_completion_test.go` - Add comprehensive test cases

Testing Patterns

Tests should follow Go testing best practices:

• Use table-driven tests for multiple scenarios
• Mock the OpenAI client where appropriate
• Use meaningful test names describing what is being tested
• Include both positive and negative test cases

References

• Implementation: `backend/model/openai_completion.go`
• Provider interface: `backend/model/provider.go`