[SPARK-52428] Harden DataFrame error handling

[rafafrdz]

Summary

• Remove 2 panic!() calls in corr() and cov(), replace with Err(SparkError::AnalysisException(...))
• Remove 3 unimplemented!() in columns(), count(), dtypes(), replace with Err(SparkError::NotYetImplemented(...))
• Replace 7 check_same_session().unwrap() with ? propagation (return types updated to Result)
• Replace expect()/unwrap() on schema responses and type downcasts with ok_or()
• Replace client.analyze().await.unwrap() with ? in persist()/unpersist()

Test plan

• cargo build passes
• cargo test unit tests pass
• cargo fmt -- --check passes