Fixed: Missing base64 dependency for Trace::Attachment

[delner]

Summary

Problem

• CI began failing on Ruby 3.4 with cannot load such file -- base64 (LoadError).
• Production users on Ruby 3.4 using Braintrust::Trace::Attachment would also encounter this error (since Nov 10, version 0.0.4)

Cause

• Dec 2024: Ruby 3.4.0 released, moving base64 from stdlib to bundled gem
• Aug 2025: VCR 6.3.0 adds base64 as a runtime dependency for Ruby 3.4 compatibility
• Nov 10, 2025: Attachment class added with require "base64" — bug introduced but hidden because VCR (dev dependency) transitively provided base64
• Dec 22, 2025: VCR 6.4.0 drops base64 dependency — bug unveiled in CI

Solution

Created Internal::Encoding::Base64 module using Ruby's native pack/unpack methods, eliminating the external dependency entirely:

Internal::Encoding::Base64.strict_encode64(data)  # uses [data].pack("m0")
Internal::Encoding::Base64.strict_decode64(str)   # uses str.unpack1("m0")

[delner]

Fun discovery: looks like this was broken upstream in openai/openai-ruby#225 too, fixed in version 0.37.0. Older versions running Ruby 3.4 will remain affected.