Fix byte-limited reads for Enumerator-backed request bodies

[fallintoplace]

Summary

• fix Enumerator-backed ReadIOAdapter#read(max_len) to return at most max_len bytes
• use byte-oriented buffer slicing for streamed request bodies
• preserve already buffered bytes when read(nil) follows a partial read
• clear out_string consistently at EOF
• add regression tests for zero-length reads, byte lengths, EOF, output buffers, and buffered reads

Fixes #274.

Test plan

• bundle exec rake TEST=test/openai/internal/util_test.rb test
• bundle exec rake lint