Encode content to base64 in UpdateFileFn (#26)

Same to CreateFileFn

Reviewed-on: https://gitea.com/gitea/gitea-mcp/pulls/26
Reviewed-by: hiifong <i@hiif.ong>
Co-authored-by: yp05327 <576951401@qq.com>
Co-committed-by: yp05327 <576951401@qq.com>

operation/repo/file.go

@@ -144,7 +144,7 @@ func UpdateFileFn(ctx context.Context, req mcp.CallToolRequest) (*mcp.CallToolRe
 
 	opt := gitea_sdk.UpdateFileOptions{
 		SHA:     sha,
-		Content: content,
+		Content: base64.StdEncoding.EncodeToString([]byte(content)),
 		FileOptions: gitea_sdk.FileOptions{
 			Message:    message,
 			BranchName: branchName,