Allow Response::Body#each to take a buffer_size

[hikmetba-bit]

Summary

Fixes #782.

Body#each always read in HTTP::Connection::BUFFER_SIZE (16KB) chunks with no way to override it, which meant streaming large responses (e.g. multi-gigabyte files via ActionController::Live) required many more reads than necessary.

Change

Adds an optional buffer_size: keyword to #each, defaulting to the existing Connection::BUFFER_SIZE so behavior is unchanged unless a caller opts in — per the implementation @ixti sketched directly in the issue:

def each(buffer_size: Connection::BUFFER_SIZE)
  while (chunk = readpartial(buffer_size))
    yield chunk
  end
end

(adapted slightly to the current readpartial(*)-forwarding signature and loop/rescue EOFError control flow already in this codebase).

Also updated the RBS signature in sig/http.rbs for Response::Body#each.

Testing

• Added two regression tests: one confirming buffer_size: is forwarded to the underlying readpartial, one confirming the default is still Connection::BUFFER_SIZE when no buffer_size: is given.
• Verified both new tests fail against the code without this fix (ArgumentError: wrong number of arguments / default nil instead of 16384), and pass with it.
• Ran all test/http/response/** and body_test.rb files: 502 runs, 0 failures, 0 errors.
• bundle exec steep check: no new type errors (only 4 pre-existing, unrelated Redundant ignore comment warnings in other files).
• bundle exec rubocop on the changed Ruby files: no offenses.

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