[feature/openrouter-provider] Refactor finish reason handling and tool call logic in agent and OpenAI provider

- Simplified finish reason check in `agent.go` by removing redundant variable assignment.
- Updated `openai.go` to override the finish reason to `FinishReasonToolUse` when tool calls are present.
- Ensured consistent finish reason handling in both `send` and `stream` methods of the OpenAI provider.

Author: isaac-scarrott

internal/llm/provider/openai.go

@@ -280,7 +280,6 @@ func (o *openaiClient) stream(ctx context.Context, messages []message.Message, t
 
 				finishReason := o.finishReason(string(acc.ChatCompletion.Choices[0].FinishReason))
 
-				// Override finish reason to tool_use if there are tool calls
 				if len(toolCalls) > 0 {
 					finishReason = message.FinishReasonToolUse
 				}