feat: add RouterState.isNavigationPending() to detect pre-navigation state

[mvanhorn]

Summary

RouterState gains isNavigationPending(), returning true while navigationTarget is still null (before the first navigation completes), so user code can detect the pre-navigation state without an NPE.

Why this matters

In #24471, dereferencing RouterState.navigationTarget() in browserless tests, or in a running app before the first navigation, throws an NPE. @mcollovati noted the Javadoc already documents the null window, making the NPE an application bug, and proposed exactly this helper so the state is easy to detect. The navigationTarget component Javadoc now references the safe check, so users discover it where they would otherwise hit the null.

Testing

RouterStateTest covers both states: pending (null target, isNavigationPending() true, currentView() empty) and completed (target set, pending false).

Fixes #24471

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[cla-assistant]

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