Fix crash when inserting newlines rapidly

[bilaldev05]

This PR fixes an assertion failure in editable_box.dart when rapidly inserting new lines.

Under fast input, the document model can grow faster than the render tree updates, causing childAtPosition() to return null.

Instead of asserting, we now gracefully fallback to the last available child.

Steps to reproduce:

1. Open demo app
2. Hold Enter to insert many new lines
3. Observe assertion error in console

After fix:
No assertion occurs and scrolling works normally.

Tested on:
Android (Pixel Fold)
Flutter 3.41.0

[codecov]

Codecov Report

❌ Patch coverage is 50.00000% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 88.10%. Comparing base (ebd507e) to head (ac696ee).
⚠️ Report is 2 commits behind head on master.

Files with missing lines	Patch %	Lines
...kages/fleather/lib/src/rendering/editable_box.dart	50.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #578      +/-   ##
==========================================
+ Coverage   85.72%   88.10%   +2.38%     
==========================================
  Files          50       63      +13     
  Lines        8096    10899    +2803     
==========================================
+ Hits         6940     9603    +2663     
- Misses       1156     1296     +140     

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