fix(build): ensure ./gradlew clean removes stale SaaS build artifacts (#7434)

### Motivation
- `./gradlew clean` on the default/proprietary flavor did not remove
`app/saas/build` left behind by earlier SaaS builds, causing stale
artifacts to persist across flavors.

### Description
- Add a `clean` hook in `build.gradle` that deletes `app/saas/build`
(`tasks.named('clean') { delete
layout.projectDirectory.dir('app/saas/build') }`) so the root `clean`
always removes SaaS artifacts even when `:saas` is not included.

### Testing
- Created `app/saas/build/clean-regression-marker`, ran `./gradlew
clean`, and verified the `app/saas/build` directory was deleted
(success).
- Ran `./gradlew spotlessCheck test`; Spotless checks passed but the
full test run reported environment-dependent unit test failures
unrelated to this change (8 failures), and `task backend:check` could
not be executed because the `task` CLI is not available in the
environment.

------
[Codex
Task](https://chatgpt.com/codex/cloud/tasks/task_e_6a7b0a9aac888325aa78e05c953355dc)

build.gradle

@@ -95,6 +95,12 @@ bootJar {
     enabled = false
 }
 
+// :saas is only included for SaaS builds, but a default-flavor clean must still remove artifacts
+// left behind by an earlier SaaS build.
+tasks.named('clean') {
+    delete layout.projectDirectory.dir('app/saas/build')
+}
+
 // Configure main class for the root project
 springBoot {
     mainClass = 'stirling.software.SPDF.SPDFApplication'