What this PR does / why we need it:

When spark.eventLog.enabled: "true" and spark.eventLog.dir points to an S3A path, feast materialize-incremental silently writes nothing to the online store and exits with code 0.
The failure chain:

SparkContext.__init__
  └─ EventLoggingListener.start()
       └─ EventLogFileWriter.requireLogBaseDirAsDirectory()
            └─ S3A 404 (prefix doesn't exist) → raises RuntimeException
                 └─ caught by _materialize_one(except Exception) → ERROR job
                      └─ CLI exits 0 — no data written, no visible error

S3 has no real directories. An empty prefix is indistinguishable from "does not exist", so Spark's pre-flight check always fails on a fresh bucket.

Which issue(s) this PR fixes:

In get_or_create_new_spark_session() (compute_engines/spark/utils.py), before building the SparkSession, call _ensure_s3a_event_log_dir() which:

1. Checks if the S3A prefix already contains objects (no-op if it does)
2. Writes a zero-byte .keep placeholder if empty
3. Uses boto3 — already a Feast dependency via the S3 offline store
4. Is fully non-fatal: swallows errors and lets Spark surface its own message if the write fails

No-ops for non-S3A paths (hdfs://, file://, etc.) and when event logging is disabled.

Checks

• I've made sure the tests are passing.
• My commits are signed off (git commit -s)
• My PR title follows conventional commits format

Testing Strategy

• Unit tests
• Integration tests
• Manual tests
• Testing is not required for this change

Misc