feat(routing): implement unified model routing with OAuth and API key providers

- Added a new routing package to manage provider registration and model resolution.
- Introduced Router, Executor, and Provider interfaces to handle different provider types.
- Implemented OAuthProvider and APIKeyProvider to support OAuth and API key authentication.
- Enhanced DefaultModelMapper to include OAuth model alias handling and fallback mechanisms.
- Updated context management in API handlers to preserve fallback models.
- Added tests for routing logic and provider selection.
- Enhanced Claude request conversion to handle reasoning content based on thinking mode.

internal/translator/openai/claude/openai_claude_request.go

@@ -61,10 +61,13 @@ func ConvertClaudeRequestToOpenAI(modelName string, inputRawJSON []byte, stream
 	out, _ = sjson.Set(out, "stream", stream)
 
 	// Thinking: Convert Claude thinking.budget_tokens to OpenAI reasoning_effort
+	// Also track if thinking is enabled to ensure reasoning_content is added for tool_calls
+	thinkingEnabled := false
 	if thinkingConfig := root.Get("thinking"); thinkingConfig.Exists() && thinkingConfig.IsObject() {
 		if thinkingType := thinkingConfig.Get("type"); thinkingType.Exists() {
 			switch thinkingType.String() {
 			case "enabled":
+				thinkingEnabled = true
 				if budgetTokens := thinkingConfig.Get("budget_tokens"); budgetTokens.Exists() {
 					budget := int(budgetTokens.Int())
 					if effort, ok := thinking.ConvertBudgetToLevel(budget); ok && effort != "" {
@@ -217,6 +220,10 @@ func ConvertClaudeRequestToOpenAI(modelName string, inputRawJSON []byte, stream
 						// Add reasoning_content if present
 						if hasReasoning {
 							msgJSON, _ = sjson.Set(msgJSON, "reasoning_content", reasoningContent)
+						} else if thinkingEnabled && hasToolCalls {
+							// Claude API requires reasoning_content in assistant messages with tool_calls
+							// when thinking mode is enabled, even if empty
+							msgJSON, _ = sjson.Set(msgJSON, "reasoning_content", "")
 						}
 
 						// Add tool_calls if present (in same message as content)