[Adityakk9031]

This PR addresses two related issues when handling IO objects during multipart form-data requests:

1. Pathless IO objects in write_multipart_chunk:

   • write_multipart_chunk matched in Pathname | IO and called ::File.basename(val.to_path).
   • For IO objects not backed by filesystem paths (such as IO.pipe, sockets, or in-memory streams), val.to_path is nil. Calling ::File.basename(nil) raised a TypeError: no implicit conversion of nil into String, crashing the request.
   • Now checks in IO unless val.to_path.nil? before extracting a filename.

2. FilePart filename basename and missing pathname require:

   • In FilePart#initialize, passing an IO (e.g. File.open(...)) stored content.to_path directly, retaining full directory paths (unlike Pathname and explicit filenames which use basename). It now normalizes via content.to_path&.then { ::File.basename(_1) }.
   • Added require "pathname" to file_part.rb to prevent NameError: uninitialized constant ContextDev::FilePart::Pathname when FilePart is loaded standalone.

Added tests for pathless IO multipart encoding and FilePart filename extraction.

cc @contextualbeer

[Adityakk9031]

Hi @contextualbeer, whenever you have a chance, could you please take a look at this PR? Thank you!