refactor: reduce code duplication through cleanup

[knight42]

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[Copilot]

Pull Request Overview

This PR refactors multipart form file handling by consolidating duplicate code into a single method. The main purpose is to reduce code duplication by moving multipart field writing logic from util.go into a method on the MultipartField struct.

Key changes:

• Consolidated addMultipartFormField and writeMultipartFormFile functions into a single writeToMultipartWriter method on MultipartField
• Added a Close method to multipartAndPipeWriter for cleaner resource management
• Increased buffer size from 512 bytes to 32KB and reused it with io.CopyBuffer for better performance

Reviewed Changes

Copilot reviewed 5 out of 5 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
multipart.go	Added writeToMultipartWriter method to MultipartField and Close method to multipartAndPipeWriter
util.go	Removed addMultipartFormField and writeMultipartFormFile functions and mime/multipart import
request.go	Updated to call the new writeToMultipartWriter method instead of addMultipartFormField
client.go	Simplified cleanup by calling mpw.Close() instead of closing writers individually
util_test.go	Updated tests to use the new method on MultipartField struct

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