Fix byte-limited reads for Enumerator-backed request bodies

[fallintoplace]

Summary

• make Enumerator-backed ReadIOAdapter#read(max_len) return at most max_len bytes
• use byte-oriented buffering so multibyte strings follow IO#read semantics
• preserve buffered bytes when an unbounded read follows a partial read
• match IO behavior for zero-length reads, EOF, negative lengths, and out_string
• add regression coverage for bounded reads, multibyte data, buffering, and output buffers

Why

This adapter feeds streamed request bodies into IO consumers such as IO.copy_stream. Returning more bytes than requested can break those consumers and corrupt multipart upload boundaries; character-oriented slicing also makes the result depend on string encoding instead of the requested byte count.

Fixes #274.

Test plan

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