Fix/multipart stray linebreak

[GitGud44]

Overview

MultipartBody.serialize() writes every string part's value with write_str_value(),
which always appends a trailing newline. That newline therefore becomes part of the value
Itself. E.g. A part meant to hold the value "helloworld" is sent as "helloworld\r\n".
Consumers that compare that value exactly would then reject it.

Fixed by writing string part values with write_bytes_value() instead, which writes raw bytes with no terminator.

Related Issue

Fixes #

Demo

Before (current main), a text/plain part with value "Hello world":

--boundary\r\nContent-Type: text/plain\r\nContent-Disposition: form-data; name="message"\r\n\r\nHello world\r\n\r\n--boundary--\r\n

After (this PR):

--boundary\r\nContent-Type: text/plain\r\nContent-Disposition: form-data; name="message"\r\n\r\nHello world\r\n--boundary--\r\n

Notes

• Only string part values are affected. bytes/file parts already go through
  write_bytes_value() and never carried the stray CRLF.
• No existing test covered a plain str part before this PR, every
  test_write_object_value* case used a JSON (Parsable) part and a bytes part, never
  a bare string one.

Testing Instructions

• cd packages/serialization/multipart && poetry install
• poetry run pytest
  ** Expect test_write_object_value_with_string_part and
  test_write_object_value_with_multiple_string_parts to pass (both fail on main
  without this fix (double CRLF before the boundary instead of one))