feat: Add LastRequestExecuting event in resumable upload

[mahendra-google]

• Add tests to ensure that event is triggered once in single chunk and multichunk upload

[gemini-code-assist]

Code Review

This pull request introduces a new LastRequestExecuting event to the ResumableUpload class, which is triggered when the final chunk of data is uploading. It also adds corresponding unit tests to verify the event's behavior in both single-chunk and multi-chunk upload scenarios. The review feedback suggests correcting a minor grammatical typo in the event's XML documentation and splitting two statements onto separate lines in the multi-chunk test file to improve readability.

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[gemini-code-assist]

There is a minor grammatical typo in the XML documentation: 'final chunk of a data' should be 'final chunk of data' or 'final chunk of the data'.

        /// <summary>
        /// Event called when the final chunk of data is uploading.
        /// </summary>

[gemini-code-assist]

For better readability and adherence to standard C# coding conventions, avoid placing multiple statements on a single line. Please split the uploader initialization and the eventInvokeCount declaration into separate lines.

                var uploader = new TestResumableUpload(service, "MultiChunk", "POST", content, "text/plain", chunkSize);
                int eventInvokeCount = 0;