Add support for string arrays in multipart fields

[Zcodeoooo]

Version
1.57

Steps to reproduce
An extension to the issue
#2784

Adds support for string arrays in multipart fields, allowing usage like:

multipart = {
    "codes[]": ["C34376202", "C481866"],
    "files"= [ 
        { "name"="Document1.txt", "mimeType":"text/plain", buffer: b"Text file 1 content" },
        { "name"="Document2.txt", "mimeType":"text/plain", buffer: b"Text file 2 content" }
    ]
}


Previously, only single values (str/file) were supported due to type and logic limitations.
Changes made
✅ Updated multipart type hint to include List[Union[str, FilePayload]]
✅ Enhanced handling of list values in form field processing
✅ Improved is_file_array_payload to correctly identify file arrays
✅ Added support for multiple same-named fields (e.g. codes[]) via list expansion
Why this is useful
Enables sending arrays of strings (e.g. batch codes, IDs) in multipart requests — a common use case when interacting with web forms or APIs expecting repeated keys.
This aligns with real-world HTTP behavior where field[] syntax is widely used for array submission.

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