Support Rails usage without an explicit `require "openai"`

[jbeckwith-oai]

Carried over from #126 and originally reported in detail by @agarcher.

The specific tapioca dsl early-return problem was fixed in 0.5.0, but the broader request in #126 was for Rails applications to be able to reference OpenAI after normal application boot without adding an application-level require "openai".

We should make this behavior explicit and regression-tested.

Desired behavior

• A minimal Rails application with gem "openai" in its Gemfile can reference OpenAI::Client after normal boot without a manual require "openai".
• The behavior continues to work when Tapioca is present and during tapioca dsl.
• The README documents the expected Rails/Bundler setup.
• Add a focused integration or smoke test so this does not regress.

Source: #126.