fix(http-client): replace Tempfile with StringIO for multipart uploads

- Replace Tempfile with StringIO in prepare_multipart_payload to eliminate filesystem dependencies
- Add enhanced StringIO compatibility methods for HTTParty multipart handling
- Update test expectations from Tempfile to StringIO
- Add defensive file closing in messages.rb
- Resolves issues with read-only containers and AWS Lambda environments

Fixes #528

Author: AaronDDM

.gitignore

@@ -2,6 +2,9 @@
 **/*/.env
 # Example test files
 examples/messages/large_test_file.txt
+examples/messages/small_pdf_test_file.pdf
+examples/messages/large_pdf_test_file.pdf
+examples/messages/large_jpg_test_file.jpg
 *.gem
 *.rbc
 .bundle

CHANGELOG.md

@@ -3,6 +3,7 @@
 ### [Unreleased]
 * Added support for `single_level` query parameter in Folders API for Microsoft accounts
 * Added support for `include_hidden_folders` query parameter in folders list endpoint for Microsoft accounts to control whether hidden folders are included in the response
+* Replace Tempfile with StringIO for multipart uploads to eliminate filesystem dependencies and improve compatibility with read-only containers and AWS Lambda environments
 
 ### 6.5.0 / 2025-06-13
 * Replaced `rest-client` dependency with `httparty` for improved maintainability and security