fix(navigation): clamp navigateViewGroup index before PageController.jumpToPage

[cursor]

Description

Fixes a crash when navigateViewGroup is invoked with a viewIndex outside the current ViewGroup tab range. PageController.jumpToPage asserts when the index is invalid; bottom navigation ViewGroups with reloadView: false supply a PageController from PageGroupWidget, so this path was reachable from app scripts.

Related Issue

None filed. Same scenario as the previously closed draft #2220 (not merged). #2225 addressed stale indices when payloads shrink, not out-of-range indices on explicit navigateViewGroup actions.

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

• In NavigateViewGroupAction.execute, resolve viewIndex through safeViewGroupPayloadIndex using the nearest PageGroup ancestor’s menu.menuItems.length, then pass that index to jumpToPage and viewGroupNotifier.updatePage.

How to Test

1. From modules/ensemble, run flutter test test/safe_view_group_payload_index_test.dart (covers the clamp helper used here).
2. Manual: bottom-nav ViewGroup with at least two tabs, call navigateViewGroup with viewIndex larger than the last tab index; the app should land on the last 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 environment did not have the Flutter SDK on PATH, so flutter analyze / flutter test were not executed here. Please run them locally per AGENTS.md.

Duplicate check

• Open PRs: Searched gh pr list and gh search prs for navigateViewGroup / jumpToPage / clampViewGroupTabIndex. No open PR fixes this NavigateViewGroupAction + jumpToPage path. Open PRs fix(storage): defer binding updates until ensemble.storage.clear() completes #2231 (storage.clear ordering) and fix(upload): complete cancelAll when some uploads already finished #2235 (upload cancelAll loop) touch different code paths and root causes.
• Closed / merged: fix(navigation): clamp navigateViewGroup viewIndex to valid tab range #2220 proposed an equivalent fix but was closed without merging. fix(navigation): clamp ViewGroup tab index when tab list shrinks #2225 merged payload index syncing when tab lists shrink; it does not clamp viewIndex in NavigateViewGroupAction.
• Branches: No other automation branch was found for this trigger; this work is on cursor/critical-bug-identification-0103 only.
• Issues: No matching ticket found for this specific crash path.

This PR reuses existing safeViewGroupPayloadIndex from page_group.dart (already unit-tested) instead of introducing a duplicate clamp helper.