build(java): bump Spark 3 from 3.5.8 to 3.5.9 (#8812)

## Rationale for this change

Spark 3.5.9 was released to Maven Central on 2026-07-12 as the latest
maintenance
release of the 3.5 line, containing bug fixes over 3.5.8. Keeping the
Spark 3
compileOnly/test dependency current picks up those fixes for the
`vortex-spark`
connector's Spark 3 builds.

## What changes are included in this PR?

A one-line version bump in `java/gradle/libs.versions.toml`: `spark3 =
"3.5.8"` →
`spark3 = "3.5.9"`. No code changes.

Verified locally against 3.5.9 with `:vortex-spark_2.12:test`: 120 tests
ran, all
passed except `VortexDataSourceS3MockTest`, which failed only because
the local Docker
daemon was not running (Testcontainers infrastructure requirement,
unrelated to the
Spark version).

## What APIs are changed? Are there any user-facing changes?

None. Spark 3.5.9 is a drop-in maintenance upgrade; no connector code or
public API
is touched.

Signed-off-by: jackylee <qcsd2011@gmail.com>

Author: jackylee-ch

java/gradle/libs.versions.toml

@@ -12,7 +12,7 @@ netty = "4.2.15.Final"
 nopen = "1.0.1"
 roaringbitmap = "1.6.14"
 slf4j = "2.0.18"
-spark3 = "3.5.8"
+spark3 = "3.5.9"
 spark4 = "4.1.1"
 s3mock = "5.0.0"
 testcontainers-jupiter = "1.21.4"