Modernize Rocket with tests, CI, and docs

[tompesman]

Summary

• Adds a high-coverage unit test suite around configuration, request handling, response parsing, supervision, and the public Rocket.push/1 API.
• Modernizes the codebase for Elixir 1.18.3 / OTP 26, replaces deprecated APIs, and normalizes request and response behavior.
• Adds CI, coverage gating, documentation, and package metadata for the current maintenance phase.

Testing

• Unit tests added for success and error paths across request, response, config, and GenStage flow.
• Local verification passed with the coverage gate above 90% and the documentation build succeeding.