fix: skip vaadinBuildFrontend in development mode

[mcollovati]

The Gradle vaadinBuildFrontend task is now skipped when Vaadin production
mode is not enabled. This prevents development and multi-module builds from
failing when the task is part of the build graph, and avoids producing a
production configuration while running in development mode.

Fixes #25000

[mcollovati]

The task is guarded with Gradle's onlyIf rather than by marking the token
service reference @Optional. This is intentional: onlyIf is evaluated before
Gradle's task input validation, so skipping in development mode avoids the
validation failure reported in #25000 (the mandatory @ServiceReference
"vaadinBuildFrontendToken" has no value because FlowPlugin registers that
build service only in production mode).

Skipping also matters beyond the crash: running vaadinBuildFrontend in
development mode would write flow-build-info.json with productionMode=true and
replace the development token, causing the running application to behave as in
production.

Verified against the issue's reproducer: with the released 25.2.2 plugin the
build fails at vaadinBuildFrontend validation; with this change the task is
SKIPPED and the jar build succeeds, with productionMode=false and the
reporter's jar.dependsOn('vaadinBuildFrontend') wiring unchanged.

[github-actions]

Test Results

 1 456 files  ±0   1 456 suites  ±0   1h 38m 2s ⏱️ - 2m 58s
10 306 tests ±0  10 238 ✅ ±0  68 💤 ±0  0 ❌ ±0 
10 778 runs  ±0  10 709 ✅ ±0  69 💤 ±0  0 ❌ ±0 

Results for commit eab02f5. ± Comparison against base commit 43f0ec7.

[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud