Expose response header metadata on SDK responses

[ktamulonis]

This change exposes response headers on successful SDK responses so callers can access values like openai-processing-ms and x-request-id for observability and
logging.

What this adds:

• response_headers on standard model responses
• response_headers on page responses
• response_headers on stream objects

Implementation notes:

• response headers were already captured internally in the transport layer
• this change attaches them to successful non-streaming response objects and page objects
• stream objects already held response headers internally; this surfaces them through a public method
• this PR intentionally keeps the API narrow and does not add convenience accessors for individual headers
• this does not change request behavior or retry behavior

Tests run:

• bundle exec ruby -Itest test/openai/client_test.rb
• bundle exec ruby -Itest test/openai/resources/responses/streaming_test.rb
• bundle exec ruby -Itest test/openai/resources/chat/completions/streaming_test.rb
• bundle exec rake test