fix(navigation): clamp ViewGroup tab index when tab list shrinks

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Description

Fixes a crash when the global viewGroupNotifier holds a tab index that is no longer valid for the current pagePayloads list (for example after _restoreVisibleScreenOnForeground reapplies a stored index from before the app backgrounded, while the live definition now exposes fewer tabs). Drawer, sidebar, and bottom-nav code paths indexed pagePayloads[viewGroupNotifier.viewIndex] and used that value for IndexedStack / PageController.initialPage without bounds checks, which produced RangeError and broke the app.

Related Issue

None filed; found by tracing _restoreVisibleScreenOnForeground in ensemble_provider.dart and the ViewGroup build paths.

Type of Change

• Bug fix (non-breaking change that fixes an issue)
• New feature (non-breaking change that adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• Documentation update
• Refactoring (no functional changes)

What Has Changed

• Added safeViewGroupPayloadIndex in page_group.dart and used it wherever the UI indexes pagePayloads or drives IndexedStack by tab index.
• When the notifier index is out of range, schedule a post-frame updatePage with the clamped index so persistence and navigation stay aligned.
• BottomNavPageGroup / PageGroupWidgetWrapper now clamp PageController.initialPage and reload-view body indexing using the same helper; PageGroupWidgetWrapper takes pageCount for clamping.
• Added unit tests for safeViewGroupPayloadIndex.

How to Test

1. From modules/ensemble, run flutter test test/safe_view_group_payload_index_test.dart.
2. Manually: use a ViewGroup with multiple tabs, select the last tab, background the app, deploy a definition with fewer tabs, resume with artifact refresh enabled so the stored index is restored; the app should show the last valid tab instead of crashing.

Screenshots / Videos

N/A

Checklist

• I have run flutter analyze and addressed any new warnings
• I have run flutter test and all tests pass
• I have tested my changes on the relevant platform(s)
• I have updated documentation if needed
• My changes do not introduce new warnings or errors

Note: The automation sandbox did not have the Flutter SDK on PATH, so flutter test / flutter analyze were not executed here. CI or a local run is appreciated.

Duplicate check

Reviewed open and recent PRs with gh pr list (EnsembleUI/ensemble). Open PR #2220 clamps navigateViewGroup action indices only; it does not cover the background-restore path or defensive indexing in PageGroupState / BottomNavPageGroup, so this is not a duplicate of that work. Open #2215 addresses ListViewCore scroll-controller swapping (unrelated). No existing branch or PR fixes the same stale-index-after-resume trigger for pagePayloads indexing.