fix: add reasoning content in the request for gcp gemini (#1603)

**Description**

This is motivated by
https://docs.cloud.google.com/vertex-ai/generative-ai/docs/thought-signatures:
```
Gemini 3 Pro enforces stricter validation on thought signatures than previous Gemini versions because they improve model performance for function calling. To ensure the model maintains full context across multiple turns of a conversation, you must return the thought signatures from previous responses in your subsequent requests. If a required thought signature is not returned when using Gemini 3 Pro, the model returns a 400 error.
```
Without this, gemini-3 can not even finish a complete tool call
procedure.

---------

Signed-off-by: yxia216 <[email protected]>

Author: hustxiayang

internal/translator/gemini_helper.go

@@ -279,6 +279,12 @@ func assistantMsgToGeminiParts(msg openai.ChatCompletionAssistantMessageParam) (
 				if contPart.Text != nil && *contPart.Text != "" {
 					parts = append(parts, genai.NewPartFromText(*contPart.Text))
 				}
+			case openai.ChatCompletionAssistantMessageParamContentTypeThinking:
+				if contPart.Text != nil && *contPart.Text != "" {
+					thoughtPart := genai.NewPartFromText(*contPart.Text)
+					thoughtPart.Thought = true
+					parts = append(parts, thoughtPart)
+				}
 			case openai.ChatCompletionAssistantMessageParamContentTypeRefusal:
 				// Refusal messages are currently ignored in this implementation.
 			default:

internal/translator/gemini_helper_test.go

@@ -319,6 +319,126 @@ func TestAssistantMsgToGeminiParts(t *testing.T) {
 			expectedParts:     nil,
 			expectedToolCalls: map[string]string{},
 		},
+		{
+			name: "thinking content with valid text",
+			msg: openai.ChatCompletionAssistantMessageParam{
+				Content: openai.StringOrAssistantRoleContentUnion{
+					Value: []openai.ChatCompletionAssistantMessageParamContent{
+						{
+							Type: openai.ChatCompletionAssistantMessageParamContentTypeThinking,
+							Text: ptr.To("Let me think step by step..."),
+						},
+					},
+				},
+				Role: openai.ChatMessageRoleAssistant,
+			},
+			expectedParts: []*genai.Part{
+				{
+					Text:    "Let me think step by step...",
+					Thought: true,
+				},
+			},
+			expectedToolCalls: map[string]string{},
+		},
+		{
+			name: "thinking content with nil text",
+			msg: openai.ChatCompletionAssistantMessageParam{
+				Content: openai.StringOrAssistantRoleContentUnion{
+					Value: []openai.ChatCompletionAssistantMessageParamContent{
+						{
+							Type: openai.ChatCompletionAssistantMessageParamContentTypeThinking,
+							Text: nil,
+						},
+					},
+				},
+				Role: openai.ChatMessageRoleAssistant,
+			},
+			expectedParts:     nil,
+			expectedToolCalls: map[string]string{},
+		},
+		{
+			name: "thinking content with empty text",
+			msg: openai.ChatCompletionAssistantMessageParam{
+				Content: openai.StringOrAssistantRoleContentUnion{
+					Value: []openai.ChatCompletionAssistantMessageParamContent{
+						{
+							Type: openai.ChatCompletionAssistantMessageParamContentTypeThinking,
+							Text: ptr.To(""),
+						},
+					},
+				},
+				Role: openai.ChatMessageRoleAssistant,
+			},
+			expectedParts:     nil,
+			expectedToolCalls: map[string]string{},
+		},
+		{
+			name: "mixed content with thinking and regular text",
+			msg: openai.ChatCompletionAssistantMessageParam{
+				Content: openai.StringOrAssistantRoleContentUnion{
+					Value: []openai.ChatCompletionAssistantMessageParamContent{
+						{
+							Type: openai.ChatCompletionAssistantMessageParamContentTypeThinking,
+							Text: ptr.To("First, I need to analyze this problem..."),
+						},
+						{
+							Type: openai.ChatCompletionAssistantMessageParamContentTypeText,
+							Text: ptr.To("Based on my analysis, here's the answer."),
+						},
+					},
+				},
+				Role: openai.ChatMessageRoleAssistant,
+			},
+			expectedParts: []*genai.Part{
+				{
+					Text:    "First, I need to analyze this problem...",
+					Thought: true,
+				},
+				genai.NewPartFromText("Based on my analysis, here's the answer."),
+			},
+			expectedToolCalls: map[string]string{},
+		},
+		{
+			name: "thinking content mixed with tool calls",
+			msg: openai.ChatCompletionAssistantMessageParam{
+				Content: openai.StringOrAssistantRoleContentUnion{
+					Value: []openai.ChatCompletionAssistantMessageParamContent{
+						{
+							Type: openai.ChatCompletionAssistantMessageParamContentTypeThinking,
+							Text: ptr.To("I need to call a function to get the weather"),
+						},
+						{
+							Type: openai.ChatCompletionAssistantMessageParamContentTypeText,
+							Text: ptr.To("Let me get the weather for you"),
+						},
+					},
+				},
+				Role: openai.ChatMessageRoleAssistant,
+				ToolCalls: []openai.ChatCompletionMessageToolCallParam{
+					{
+						ID: ptr.To("call_weather"),
+						Function: openai.ChatCompletionMessageToolCallFunctionParam{
+							Name:      "get_weather",
+							Arguments: `{"location":"San Francisco"}`,
+						},
+						Type: openai.ChatCompletionMessageToolCallTypeFunction,
+					},
+				},
+			},
+			expectedParts: []*genai.Part{
+				genai.NewPartFromFunctionCall("get_weather", map[string]any{
+					"location": "San Francisco",
+				}),
+				{
+					Text:    "I need to call a function to get the weather",
+					Thought: true,
+				},
+				genai.NewPartFromText("Let me get the weather for you"),
+			},
+			expectedToolCalls: map[string]string{
+				"call_weather": "get_weather",
+			},
+		},
 	}
 
 	for _, tc := range tests {