Fix selectSparkVersion() crash on non-SemVer runtime keys

selectSparkVersion(latest) sorts candidate runtime keys with SemVer.parse(),
which throws IllegalArgumentException on any key that is not a valid SemVer.
The clusters API started returning "v18.x-scala2.13" (two version segments and
a leading "v"), which broke version selection for every caller and turned the
nightly ClustersIT.latestRuntime red across all clouds.

Make the sort non-throwing and rank unparseable keys lowest, mirroring
databricks-sdk-go (golang.org/x/mod/semver.Compare), which ignores such keys
for "latest" rather than failing. Add SemVer.parseOrNull() and regression tests.

Author: Divyansh-db

databricks-sdk-java/src/main/java/com/databricks/sdk/mixin/ClustersExt.java

@@ -63,11 +63,32 @@ public String selectSparkVersion(SparkVersionSelector selector) throws IllegalAr
       if (!selector.latest) {
         throw new IllegalArgumentException("spark versions query returned multiple results");
       }
-      versions.sort((v1, v2) -> SemVer.parse(v2).compareTo(SemVer.parse(v1)));
+      versions.sort(ClustersExt::compareSparkVersionsDescending);
     }
     return versions.get(0);
   }
 
+  /**
+   * Compares two Spark runtime keys so that the latest version sorts first. Mirrors the
+   * databricks-sdk-go behavior (golang.org/x/mod/semver.Compare), where keys that are not valid
+   * SemVer (for example "v18.x-scala2.13") are treated as lowest priority instead of throwing. This
+   * ensures a single unparseable runtime key returned by the API cannot break version selection.
+   */
+  private static int compareSparkVersionsDescending(String v1, String v2) {
+    SemVer s1 = SemVer.parseOrNull(v1);
+    SemVer s2 = SemVer.parseOrNull(v2);
+    if (s1 == null && s2 == null) {
+      return 0;
+    }
+    if (s1 == null) {
+      return 1; // v1 is unparseable: sort it after v2
+    }
+    if (s2 == null) {
+      return -1; // v2 is unparseable: keep v1 before v2
+    }
+    return s2.compareTo(s1); // descending order: latest first
+  }
+
   public String selectNodeType(NodeTypeSelector selector) {
     // Logic ported from
     // https://github.com/databricks/databricks-sdk-go/blob/main/service/clusters/node_type.go

databricks-sdk-java/src/main/java/com/databricks/sdk/mixin/SemVer.java

@@ -47,6 +47,19 @@ public static SemVer parse(String v) {
         m.group("build"));
   }
 
+  /**
+   * Parses the given version string, returning {@code null} instead of throwing when it is not a
+   * valid SemVer. Useful when sorting collections that may contain non-SemVer values (for example
+   * Spark runtime keys such as "v18.x-scala2.13").
+   */
+  public static SemVer parseOrNull(String v) {
+    try {
+      return parse(v);
+    } catch (IllegalArgumentException e) {
+      return null;
+    }
+  }
+
   @Override
   public int compareTo(SemVer other) {
     if (other == null) {

databricks-sdk-java/src/test/java/com/databricks/sdk/mixin/ClustersExtTest.java

@@ -143,4 +143,34 @@ void sparkVersionWithSparkVersionParameter() {
         clustersExt.selectSparkVersion(new SparkVersionSelector().withSparkVersion("3.4.1"));
     assertEquals("13.3.x-scala2.12", sparkVersion);
   }
+
+  private GetSparkVersionsResponse testGetSparkVersionsWithUnparseableKey() {
+    Collection<SparkVersion> versions = new ArrayList<>();
+    versions.add(
+        new SparkVersion()
+            .setName("14.3 LTS (includes Apache Spark 3.5.0, Scala 2.12)")
+            .setKey("14.3.x-scala2.12"));
+    versions.add(
+        new SparkVersion()
+            .setName("15.4 LTS (includes Apache Spark 3.5.0, Scala 2.12)")
+            .setKey("15.4.x-scala2.12"));
+    // Non-SemVer runtime key returned by the API. Sorting this with SemVer.parse() previously threw
+    // "Not a valid SemVer: v18.x-scala2.13" and broke selection for every caller.
+    versions.add(
+        new SparkVersion()
+            .setName("18.x (includes Apache Spark 4.0.0, Scala 2.13)")
+            .setKey("v18.x-scala2.13"));
+    return new GetSparkVersionsResponse().setVersions(versions);
+  }
+
+  @Test
+  void selectLatestSparkVersionIgnoresUnparseableKey() {
+    ClustersExt clustersExt = new ClustersExt(clustersMock);
+    Mockito.doReturn(testGetSparkVersionsWithUnparseableKey()).when(clustersMock).sparkVersions();
+
+    // Must not throw on the non-SemVer key, and must return the latest parseable runtime - matching
+    // databricks-sdk-go, which ranks unparseable keys lowest rather than failing.
+    String sparkVersion = clustersExt.selectSparkVersion(new SparkVersionSelector().withLatest());
+    assertEquals("15.4.x-scala2.12", sparkVersion);
+  }
 }

databricks-sdk-java/src/test/java/com/databricks/sdk/mixin/SemVerTest.java

@@ -37,4 +37,21 @@ void parseTest() {
     int compareResult = parsedSemVer.compareTo(expectedSemVer);
     assertEquals(0, compareResult);
   }
+
+  @Test
+  void parseOrNullReturnsNullForInvalid() {
+    // Spark runtime key shape that crashed selectSparkVersion(): only two version segments and a
+    // leading "v", which is not a valid SemVer.
+    assertNull(SemVer.parseOrNull("v18.x-scala2.13"));
+    assertNull(SemVer.parseOrNull("not-a-version"));
+    assertNull(SemVer.parseOrNull(null));
+    assertNull(SemVer.parseOrNull(""));
+  }
+
+  @Test
+  void parseOrNullParsesValid() {
+    SemVer parsed = SemVer.parseOrNull("v1.2.3-alpha+build-20230510");
+    assertNotNull(parsed);
+    assertEquals(0, parsed.compareTo(new SemVer(1, 2, 3, "alpha", "build-20230510")));
+  }
 }