feat: add gzip batch compression

[marandaneto]

💡 Motivation and Context

Fixes #201.

The Ruby SDK sent batch payloads uncompressed, while other PostHog SDKs support request-body compression for ingestion uploads. This enables gzip compression by default for Ruby batch uploads, sets Content-Encoding: gzip, and preserves delivery by falling back to uncompressed JSON if local gzip compression fails before sending. Users can opt out with compress_request: false.

💚 How did you test it?

• bundle exec rspec
• bundle exec rubocop lib/posthog/client.rb lib/posthog/send_worker.rb lib/posthog/transport.rb spec/posthog/client_spec.rb spec/posthog/transport_spec.rb
• bundle exec rake public_api:check

📝 Checklist

• I reviewed the submitted code.
• I added tests to verify the changes.
• I updated the docs if needed.
• No breaking change or entry added to the changelog.

If releasing new changes

• Ran pnpm changeset to generate a changeset file

🤖 Agent context

Autonomy: Human-driven (agent-assisted)

Implemented with pi. I checked the existing SDK gzip behavior for the expected header-based compression, then made Ruby batch uploads gzip by default. The implementation uses Ruby stdlib gzip support, sets Content-Encoding: gzip without adding a compression query parameter, and falls back to the original uncompressed body only for Zlib::Error compression failures so events are not dropped.

[greptile-apps]

Comments Outside Diff (1)

1. lib/posthog/transport.rb, line 153-155 (link)

   The stub debug log still references @path (the original constant path) after the refactor, so when compression is enabled the log will show /batch/ instead of the actual /batch/?compression=gzip path that would be used for a real request. request_path is already in scope and captures the correct value.

   Note: If this suggestion doesn't match your team's coding style, reply to this and let me know. I'll remember it for next time!

_{Reviews (1): Last reviewed commit: "feat: add gzip batch compression" | Re-trigger Greptile}

[marandaneto]

Addressed Greptile feedback in f40aedc: simplified compress_request option forwarding in Client/SendWorker, and updated the stub debug log to reference the computed request path.

[greptile-apps]

_{Reviews (2): Last reviewed commit: "feat: add gzip batch compression" | Re-trigger Greptile}

[marandaneto]

@turnipdabeets can you review this one pls cus i am blocked here #200 (comment)