fix: fall back to caret offset when a text line yields no glyph boxes (selection endpoints & line boundary)

[mtallenca]

This PR contains two closely-related crash fixes in text_line.dart. Both stem from the same root cause — code that reads glyph-box geometry and assumes getBoxesForSelection returns at least one box — and both apply the same remedy: fall back to the caret offset when there are no boxes.

Fix 1 — selection endpoints (_getEndpointForSelection)

RenderEditableTextLine._getEndpointForSelection assumes getBoxesForSelection always returns at least one box for a non-collapsed selection:

final boxes = _getBoxes(textSelection);
assert(boxes.isNotEmpty);
final targetBox = first ? boxes.first : boxes.last;

That assumption doesn't hold when a selection edge lands on a position that produces no glyph boxes — for example an offset inside a grapheme cluster (emoji surrogate pair). When that happens, boxes.first/boxes.last throws during RenderEditor.paint → getEndpointsForSelection: an assert failure in debug and a fatal RangeError in release.

Production crash (Crashlytics, flutter_quill 11.5.1, iOS):

Fatal Exception: FlutterError
0  _Array.last (dart:core)
1  RenderEditableTextLine._getEndpointForSelection (text_line.dart)
2  RenderEditableTextLine.getExtentEndpointForSelection (text_line.dart)
3  RenderEditor.getEndpointsForSelection (editor.dart)
4  RenderEditor.paint (editor.dart)

Fix: when boxes is empty, fall back to the caret offset for that selection edge — the same geometry the method already uses for collapsed selections — instead of throwing.

Fix 2 — line boundary (getLineBoundary)

getLineBoundary derives the line's left/right edges from lineBoxes.first / lineBoxes.last. An empty line (Line.length == 1) makes the [0, line.length - 1] == [0, 0] box selection collapsed, so getBoxesForSelection returns no boxes and lineBoxes ends up empty. It then threw Bad state: No element during line-boundary navigation (Home / End / Shift+Home on a hardware keyboard) with the caret on an empty line.

Fix: mirror the empty-boxes guard from Fix 1 — when lineBoxes is empty, fall back to the caret position (TextRange(start: position.offset, end: position.offset)) instead of reading .first / .last.

Tests

Added regression tests to test/bug_fix_test.dart:

• Fix 1 drives RenderEditor.getEndpointsForSelection with a selection edge inside an emoji surrogate pair — fails the boxes.isNotEmpty assert without the fix, passes with it.
• Fix 2 drives getLineBoundary on an empty line — throws Bad state: No element without the fix, passes with it.