ci(s3tables): stop Lakekeeper flaking on Docker Hub pull timeouts (#9920)

* ci(s3tables): drop docker pre-pull from Lakekeeper job

The lakekeeper repro is pure Go against the local weed binary; the job
kept failing on Docker Hub timeouts pulling python:3 and localstack
images the test never runs. Also drop the stale python-in-docker
comments left from the old harness.

* ci(s3tables): serve python:3 from GHA cache in the STS job

Retried pulls still die when both mirror.gcr.io and registry-1.docker.io
are unreachable from the runner. Cache the saved image tarball under a
weekly key: an exact hit skips the registry entirely, a miss pulls fresh
and refreshes the cache, and a stale tarball from a previous week is the
fallback when Docker Hub is down.

* ci(spark): pre-pull the spark tag the test actually runs

The workflow warmed apache/spark:3.5.8 with retries while the
testcontainers setup runs apache/spark:3.5.1, so the real image was
pulled at test time with no retry at all.

test/s3tables/lakekeeper/lakekeeper_test.go

@@ -68,7 +68,6 @@ func TestLakekeeperIntegration(t *testing.T) {
 	env.StartSeaweedFS(t)
 	fmt.Printf(">>> SeaweedFS started.\n")
 
-	// Run python script in docker to test STS and S3 operations
 	runLakekeeperRepro(t, env)
 }
 
@@ -80,7 +79,6 @@ func TestLakekeeperTableBucketIntegration(t *testing.T) {
 	env.StartSeaweedFS(t)
 	fmt.Printf(">>> SeaweedFS started.\n")
 
-	// Run python script in docker to test STS and S3 Tables operations
 	runLakekeeperTableBucketRepro(t, env)
 }