Add system instruction support and enhance internal API handlers

- Introduced `SystemInstruction` field in `PrepareRequest` and `GenerateContentRequest` for better message parsing.
- Updated `SendMessage` and `SendMessageStream` to handle system instructions in client API calls.
- Enhanced error handling and manual flushing logic in response flows.
- Added new internal API endpoints `/v1internal:generateContent` and `/v1internal:streamGenerateContent`.
- Improved proxy handling and transport logic in HTTP client initialization.

internal/client/client.go

@@ -266,6 +266,12 @@ func (c *Client) APIRequest(ctx context.Context, endpoint string, body interface
 			url = url + "?alt=sse"
 		}
 		jsonBody = []byte(gjson.GetBytes(jsonBody, "request").Raw)
+		systemInstructionResult := gjson.GetBytes(jsonBody, "systemInstruction")
+		if systemInstructionResult.Exists() {
+			jsonBody, _ = sjson.SetRawBytes(jsonBody, "system_instruction", []byte(systemInstructionResult.Raw))
+			jsonBody, _ = sjson.DeleteBytes(jsonBody, "systemInstruction")
+			jsonBody, _ = sjson.DeleteBytes(jsonBody, "session_id")
+		}
 	}
 
 	// log.Debug(string(jsonBody))
@@ -303,15 +309,14 @@ func (c *Client) APIRequest(ctx context.Context, endpoint string, body interface
 			}
 		}()
 		bodyBytes, _ := io.ReadAll(resp.Body)
-
 		return nil, &ErrorMessage{resp.StatusCode, fmt.Errorf(string(bodyBytes))}
 	}
 
 	return resp.Body, nil
 }
 
-// SendMessageStream handles a single conversational turn, including tool calls.
-func (c *Client) SendMessage(ctx context.Context, rawJson []byte, model string, contents []Content, tools []ToolDeclaration) ([]byte, *ErrorMessage) {
+// SendMessage handles a single conversational turn, including tool calls.
+func (c *Client) SendMessage(ctx context.Context, rawJson []byte, model string, systemInstruction *Content, contents []Content, tools []ToolDeclaration) ([]byte, *ErrorMessage) {
 	request := GenerateContentRequest{
 		Contents: contents,
 		GenerationConfig: GenerationConfig{
@@ -320,6 +325,9 @@ func (c *Client) SendMessage(ctx context.Context, rawJson []byte, model string,
 			},
 		},
 	}
+
+	request.SystemInstruction = systemInstruction
+
 	request.Tools = tools
 
 	requestBody := map[string]interface{}{
@@ -402,7 +410,7 @@ func (c *Client) SendMessage(ctx context.Context, rawJson []byte, model string,
 }
 
 // SendMessageStream handles a single conversational turn, including tool calls.
-func (c *Client) SendMessageStream(ctx context.Context, rawJson []byte, model string, contents []Content, tools []ToolDeclaration) (<-chan []byte, <-chan *ErrorMessage) {
+func (c *Client) SendMessageStream(ctx context.Context, rawJson []byte, model string, systemInstruction *Content, contents []Content, tools []ToolDeclaration) (<-chan []byte, <-chan *ErrorMessage) {
 	dataTag := []byte("data: ")
 	errChan := make(chan *ErrorMessage)
 	dataChan := make(chan []byte)
@@ -418,6 +426,9 @@ func (c *Client) SendMessageStream(ctx context.Context, rawJson []byte, model st
 				},
 			},
 		}
+
+		request.SystemInstruction = systemInstruction
+
 		request.Tools = tools
 
 		requestBody := map[string]interface{}{
@@ -519,6 +530,117 @@ func (c *Client) SendMessageStream(ctx context.Context, rawJson []byte, model st
 	return dataChan, errChan
 }
 
+// SendRawMessage handles a single conversational turn, including tool calls.
+func (c *Client) SendRawMessage(ctx context.Context, rawJson []byte) ([]byte, *ErrorMessage) {
+	rawJson, _ = sjson.SetBytes(rawJson, "project", c.GetProjectID())
+
+	modelResult := gjson.GetBytes(rawJson, "model")
+	model := modelResult.String()
+	modelName := model
+	for {
+		if c.isModelQuotaExceeded(modelName) {
+			if c.cfg.QuotaExceeded.SwitchPreviewModel && c.glAPIKey == "" {
+				modelName = c.getPreviewModel(model)
+				if modelName != "" {
+					log.Debugf("Model %s is quota exceeded. Switch to preview model %s", model, modelName)
+					rawJson, _ = sjson.SetBytes(rawJson, "model", modelName)
+					continue
+				}
+			}
+			return nil, &ErrorMessage{
+				StatusCode: 429,
+				Error:      fmt.Errorf(`{"error":{"code":429,"message":"All the models of '%s' are quota exceeded","status":"RESOURCE_EXHAUSTED"}}`, model),
+			}
+		}
+
+		respBody, err := c.APIRequest(ctx, "generateContent", rawJson, false)
+		if err != nil {
+			if err.StatusCode == 429 {
+				now := time.Now()
+				c.modelQuotaExceeded[modelName] = &now
+				if c.cfg.QuotaExceeded.SwitchPreviewModel && c.glAPIKey == "" {
+					continue
+				}
+			}
+			return nil, err
+		}
+		delete(c.modelQuotaExceeded, modelName)
+		bodyBytes, errReadAll := io.ReadAll(respBody)
+		if errReadAll != nil {
+			return nil, &ErrorMessage{StatusCode: 500, Error: errReadAll}
+		}
+		return bodyBytes, nil
+	}
+}
+
+// SendRawMessageStream handles a single conversational turn, including tool calls.
+func (c *Client) SendRawMessageStream(ctx context.Context, rawJson []byte) (<-chan []byte, <-chan *ErrorMessage) {
+	dataTag := []byte("data: ")
+	errChan := make(chan *ErrorMessage)
+	dataChan := make(chan []byte)
+	go func() {
+		defer close(errChan)
+		defer close(dataChan)
+
+		rawJson, _ = sjson.SetBytes(rawJson, "project", c.GetProjectID())
+
+		modelResult := gjson.GetBytes(rawJson, "model")
+		model := modelResult.String()
+		modelName := model
+		var stream io.ReadCloser
+		for {
+			if c.isModelQuotaExceeded(modelName) {
+				if c.cfg.QuotaExceeded.SwitchPreviewModel && c.glAPIKey == "" {
+					modelName = c.getPreviewModel(model)
+					if modelName != "" {
+						log.Debugf("Model %s is quota exceeded. Switch to preview model %s", model, modelName)
+						rawJson, _ = sjson.SetBytes(rawJson, "model", modelName)
+						continue
+					}
+				}
+				errChan <- &ErrorMessage{
+					StatusCode: 429,
+					Error:      fmt.Errorf(`{"error":{"code":429,"message":"All the models of '%s' are quota exceeded","status":"RESOURCE_EXHAUSTED"}}`, model),
+				}
+				return
+			}
+			var err *ErrorMessage
+			stream, err = c.APIRequest(ctx, "streamGenerateContent", rawJson, true)
+			if err != nil {
+				if err.StatusCode == 429 {
+					now := time.Now()
+					c.modelQuotaExceeded[modelName] = &now
+					if c.cfg.QuotaExceeded.SwitchPreviewModel && c.glAPIKey == "" {
+						continue
+					}
+				}
+				errChan <- err
+				return
+			}
+			delete(c.modelQuotaExceeded, modelName)
+			break
+		}
+
+		scanner := bufio.NewScanner(stream)
+		for scanner.Scan() {
+			line := scanner.Bytes()
+			if bytes.HasPrefix(line, dataTag) {
+				dataChan <- line[6:]
+			}
+		}
+
+		if errScanner := scanner.Err(); errScanner != nil {
+			errChan <- &ErrorMessage{500, errScanner}
+			_ = stream.Close()
+			return
+		}
+
+		_ = stream.Close()
+	}()
+
+	return dataChan, errChan
+}
+
 func (c *Client) isModelQuotaExceeded(model string) bool {
 	if lastExceededTime, hasKey := c.modelQuotaExceeded[model]; hasKey {
 		duration := time.Now().Sub(*lastExceededTime)