chore: avoid adjust the original input obj

[johnchen05]

For long time, I was puzzled on: in our UT, when we spy on logMessage, we never get the properties _error

I thought it should be the test engine(jest, vitest) issue. but no matter what engine I use, this keeps happening.
recent days, I fork and read the cf-nodejs-logging-support code. I found the root cause.
the _error is deleted from the lastArg obj. then when the input args is checked, _error has not been inside.

When I remove this "delete" from my node_module code, my UT case pass.

I went through your whole logic, I think this delete lastArg._error should not be necessary.

would you mind I remove this line as a fix?

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