fix: percent-encode parentheses in filename* per RFC 5987 - #607

Summary

Percent-encode parentheses in filename* Content-Disposition header values per RFC 5987.

Problem

Parentheses ( and ) are separator characters per RFC 2616 and must be percent-encoded in filename* values per RFC 5987. Currently they are passed through unencoded, causing .NET backends (and other strict HTTP parsers) to reject the multipart request.

Solution

Add parentheses to the set of characters that are percent-encoded when generating filename* values. When a filename contains characters outside the RFC 5987 attr-char set, a filename* parameter with proper percent-encoding is emitted alongside the existing filename parameter for backward compatibility.

Test Plan

• Filename test(1).txt produces filename*=utf-8''test%281%29.txt
• Simple filenames (e.g. test.txt) do not get a filename* parameter
• Spaces are encoded (e.g. my file.txt → my%20file.txt)
• Existing tests pass
• No breaking changes to filename (non-starred) header

Fixes #572