[TextBoxView] Crash during large selection while background layout is incomplete

# [TextBoxView] Crash during large selection while background layout is incomplete

## Summary
`TextBoxView.GetLineIndexFromOffset` can fail during large-text selection when layout is still in time-sliced background progress, causing a crash/fail-fast path instead of graceful handling.

## Observed behavior
- Selecting text in very large `TextBox` content (for example, very long lines / high offsets) can trigger a consistency/assert failure.
- The failure happens when selection/highlight processing requests an offset beyond the currently measured line range.

## Repro notes (public)
1. Create/open a WPF app view with a `TextBox` containing a very large amount of text (for example, one very long line or total offsets in 100K+ range).
2. Trigger selection operations while layout is still being processed incrementally (IME/search/focus interactions can increase likelihood).
3. Observe crash/fail-fast behavior from `TextBoxView.GetLineIndexFromOffset` path.

## Impact
- Affects reliability of text editing/selection in large-document scenarios.
- More likely in interactive flows where layout and selection/focus updates interleave.

## Expected behavior
Selection/highlight should remain stable and never crash, even when background layout has not measured all lines yet.

## Actual behavior
The lookup assumes the requested offset is always inside measured `_lineMetrics`, and can fail when it is temporarily outside that range.

## Suspected root cause
`TextBoxView` performs time-sliced layout, so `_lineMetrics` may be only partially populated. During that window, selection/highlight can request an offset greater than the last measured line `EndOffset`, and the search path fails.

## Proposed fix direction
- Add defensive clamping in `GetLineIndexFromOffset` for offsets beyond currently measured range.
- Return the last measured line index when background layout is pending.
- Keep correctness checks for unexpected conditions while avoiding crash in expected incremental-layout windows.
- Remove redundant preconditions that incorrectly assume full containment during transitional layout states.