feat(config): 新增 Gemini 配置项和 geminicli 核心包

- 添加 Gemini OAuth 配置结构
- 实现 geminicli 包（OAuth、Token、CodeAssist 类型）
- 更新配置示例文件

backend/internal/pkg/geminicli/sanitize.go

@@ -0,0 +1,46 @@
+package geminicli
+
+import "strings"
+
+const maxLogBodyLen = 2048
+
+func SanitizeBodyForLogs(body string) string {
+	body = truncateBase64InMessage(body)
+	if len(body) > maxLogBodyLen {
+		body = body[:maxLogBodyLen] + "...[truncated]"
+	}
+	return body
+}
+
+func truncateBase64InMessage(message string) string {
+	const maxBase64Length = 50
+
+	result := message
+	offset := 0
+	for {
+		idx := strings.Index(result[offset:], ";base64,")
+		if idx == -1 {
+			break
+		}
+		actualIdx := offset + idx
+		start := actualIdx + len(";base64,")
+
+		end := start
+		for end < len(result) && isBase64Char(result[end]) {
+			end++
+		}
+
+		if end-start > maxBase64Length {
+			result = result[:start+maxBase64Length] + "...[truncated]" + result[end:]
+			offset = start + maxBase64Length + len("...[truncated]")
+			continue
+		}
+		offset = end
+	}
+
+	return result
+}
+
+func isBase64Char(c byte) bool {
+	return (c >= 'A' && c <= 'Z') || (c >= 'a' && c <= 'z') || (c >= '0' && c <= '9') || c == '+' || c == '/' || c == '='
+}