[SPARK-58893][UI] Fix Spark UI multi-cluster reverse proxy: propagate…

[Nflrijal]

What changes were proposed in this pull request?

When spark.ui.reverseProxy=true is enabled in Standalone multi-cluster deployments:

1. SparkContext on the driver did not receive UI_REVERSE_PROXY_URL, causing driver UI links to drop /proxy/<id> prefixes. This PR propagates UI_REVERSE_PROXY_URL (or falls back to spark.ui.proxyBase) into system property spark.ui.proxyBase.
2. Master did not pass -Dspark.ui.reverseProxyUrl=<url> to DriverDescription javaOpts. This PR updates Master to inject reverseProxyUrl into driver javaOpts when reverse proxy is enabled.
3. /app/kill and /driver/kill redirect Location headers were redirecting to / instead of reverseProxyUrl + "/". This PR updates MasterWebUI redirect handlers to preserve reverse proxy paths.
4. UIUtils.prependBaseUri and uiRoot were updated to handle request = null safely.

Note: Please backport to master, branch-4.1, and branch-3.5 if applicable.

Why are the changes needed?

Without these changes, multi-cluster reverse proxy deployments lose proxy prefixes when accessing driver UIs or terminating applications/drivers from Master UI, breaking navigation and reverse proxy routing.

Does this PR introduce any user-facing change?

No API changes. Fixes UI navigation links and redirect Location headers when spark.ui.reverseProxy=true is enabled.

How was this patch tested?

• Added unit tests in UIUtilsSuite, MasterSuite, MasterWebUISuite, and SparkContextSuite.
• Verified local build and unit tests using build/sbt "core/testOnly org.apache.spark.ui.UIUtilsSuite org.apache.spark.deploy.master.MasterSuite org.apache.spark.deploy.master.ui.MasterWebUISuite org.apache.spark.SparkContextSuite".

Was this patch authored or co-authored using generative AI tooling?

No.

[Nflrijal]

Hi @dongjoon-hyun,
Can you please take a look when you get a chance? Thanks!