[Bug] SparkProcessBuilder fails to load spark.master from spark-defaults.conf

### Code of Conduct

- [x] I agree to follow this project's [Code of Conduct](https://www.apache.org/foundation/policies/conduct)

### Search before asking

- [x] I have searched in the [issues](https://github.com/apache/kyuubi/issues?q=is%3Aissue) and found no similar issues.

### Describe the bug

The Spark engine fails to start because Kyuubi selects the wrong cluster manager, even though `spark.master` is configured in `spark-defaults.conf`.

`org.apache.kyuubi.engine.spark.SparkProcessBuilder#defaultsConf` is always empty. As a result, `SparkProcessBuilder` cannot load `spark.master` from `spark-defaults.conf`.

Since `clusterManager()` obtains the cluster manager through `getSparkOption(MASTER_KEY)`, the missing configuration causes Kyuubi to use the wrong cluster manager when managing the Spark engine application. The engine application is then reported as `NOT_FOUND`, and engine startup fails.

error log:

```
2026-08-06 18:07:28.615 ERROR LaunchEngine: Error operating LaunchEngine:
The engine application has been terminated. Please check the engine log.
ApplicationInfo: (
name -> null,
id -> null,
error -> null,
state -> NOT_FOUND,
url -> null
)

org.apache.kyuubi.KyuubiSQLException:
The engine application has been terminated. Please check the engine log.
ApplicationInfo: (
name -> null,
id -> null,
error -> null,
state -> NOT_FOUND,
url -> null
)

at org.apache.kyuubi.engine.EngineRef.$anonfun$create$11(EngineRef.scala:340)
at org.apache.kyuubi.engine.EngineRef.$anonfun$create$11$adapted(EngineRef.scala:329)
at scala.Option.foreach(Option.scala:437)
at org.apache.kyuubi.engine.EngineRef.$anonfun$create$10(EngineRef.scala:329)
at org.apache.kyuubi.engine.EngineRef.$anonfun$create$10$adapted(EngineRef.scala:318)
at scala.Option.foreach(Option.scala:437)
at org.apache.kyuubi.engine.EngineRef.$anonfun$create$1(EngineRef.scala:318)
at org.apache.kyuubi.ha.client.zookeeper.ZookeeperDiscoveryClient.tryWithLock(ZookeeperDiscoveryClient.scala:166)
at org.apache.kyuubi.engine.EngineRef.tryWithLock(EngineRef.scala:221)
at org.apache.kyuubi.engine.EngineRef.create(EngineRef.scala:226)
at org.apache.kyuubi.engine.EngineRef.$anonfun$getOrCreate$1(EngineRef.scala:377)
at scala.Option.getOrElse(Option.scala:201)
at org.apache.kyuubi.engine.EngineRef.getOrCreate(EngineRef.scala:377)
at org.apache.kyuubi.session.KyuubiSessionImpl.$anonfun$openEngineSession$2(KyuubiSessionImpl.scala:160)
at org.apache.kyuubi.session.KyuubiSessionImpl.$anonfun$openEngineSession$2$adapted(KyuubiSessionImpl.scala:134)
at org.apache.kyuubi.ha.client.DiscoveryClientProvider$.withDiscoveryClient(DiscoveryClientProvider.scala:36)
at org.apache.kyuubi.session.KyuubiSessionImpl.$anonfun$openEngineSession$1(KyuubiSessionImpl.scala:134)
at org.apache.kyuubi.session.KyuubiSession.handleSessionException(KyuubiSession.scala:49)
at org.apache.kyuubi.session.KyuubiSessionImpl.openEngineSession(KyuubiSessionImpl.scala:134)
at org.apache.kyuubi.operation.LaunchEngine.$anonfun$runInternal$1(LaunchEngine.scala:60)
at java.base/java.util.concurrent.Executors$RunnableAdapter.call(Executors.java:539)
at java.base/java.util.concurrent.FutureTask.run(FutureTask.java:264)
at java.base/java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1136)
at java.base/java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:635)
at java.base/java.lang.Thread.run(Thread.java:840)
```

The issue may have been introduced by [#6008](https://github.com/apache/kyuubi/pull/6008) due to accessing subclass state during initialization.

As shown in the screenshot, `sparkHome` is still `null` when `SparkProcessBuilder#defaultsConf` is evaluated because it has not been initialized yet. Consequently, the path to `spark-defaults.conf` is resolved incorrectly, causing `defaultsConf` to be empty and preventing `spark.master` from being loaded.

### Affects Version(s)

master

### Kyuubi Server Log Output

```logtalk

```

### Kyuubi Engine Log Output

```logtalk

```

### Kyuubi Server Configurations

```yaml

```

### Kyuubi Engine Configurations

```yaml

```

### Additional context

_No response_

### Are you willing to submit PR?

- [x] Yes. I would be willing to submit a PR with guidance from the Kyuubi community to fix.
- [ ] No. I cannot submit a PR at this time.