Remove duplicate push logging and preserve structured request results

[tompesman]

Summary

• Stop logging failed push events twice by letting Rocket.Request own request/error logging.
• Encode payloads before posting so request errors stay structured and custom response handlers still receive the original payload.
• Add coverage for configuration, encoding, exit, and HTTP error paths, including custom call/3 handlers.
• Bump locked HTTP dependency versions in mix.lock.

Testing

• Added and updated unit tests around Rocket.Request.perform/2 and Rocket.Pusher.handle_events/3.
• Verified error logging happens once per failure path and that custom response handlers are invoked exactly once.
• Not run (not requested).