[SPARK-52428] Add comprehensive CI workflow with unit and integration tests

[rafafrdz]

Summary

Add a new ci.yml workflow that improves on the existing build.yml with:

Jobs

Job	What it does	Needs Spark?
lint	cargo fmt --check + cargo clippy -D warnings (default + all features)	No
unit-tests	Build + test across feature matrix (4 combinations)	No
docs	cargo doc with -D warnings	No
integration-tests	Full test suite against Docker Spark Connect server	Yes

Improvements over existing build.yml

• Feature matrix testing — tests default, polars, datafusion, and combined features separately
• Smart unit test detection — distinguishes transport errors (expected without Spark) from real failures
• Server readiness check — waits up to 60s for Spark Connect with nc probe before running integration tests
• Failure diagnostics — dumps docker compose logs on integration test failure
• Updated actions — uses arduino/setup-protoc@v3 (not v2), dtolnay/rust-toolchain@stable
• Triggers on dev branch — supports the fork's integration branch

Test plan

• Workflow YAML is valid
• Covers all feature combinations
• Integration tests wait for server readiness
• Failure diagnostics included