feat: support multiple file uploads with the same field name

[gumruyanzh]

Summary

• Adds request.files(name) to retrieve all files matching a field name from multipart form data
• Handles <input type="file" multiple> where several files share the same field name

Problem

When an HTML form uses <input type="file" name="attachments" multiple>, the multipart request body contains multiple parts with the same field name but different filenames. The existing request.file("attachments") only returns the first match because MultipartQuery.getFile() returns on the first hit.

Changes

• MultipartQuery.zig: Add getFiles() method that collects all files with a matching key
• Request.zig: Add files() method as the public API wrapper

The existing request.file(name) is unchanged for backward compatibility.

Usage

// Single file (existing API, unchanged):
if (try request.file("upload")) |f| { ... }

// Multiple files (new API):
const uploaded = try request.files("attachments");
for (uploaded) |f| {
    // f.filename, f.content
}

Test plan

• zig build succeeds
• zig build test --summary all - all tests pass
• Existing single-file upload tests continue to pass

Closes #182

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