* Fix TextBox validation error persisting when reverting to same valid value
When a TwoWay binding with validation had an error and the user set the
target back to the same value that was last successfully written to the
source, WriteTargetValueToSource() would bail out early because the
target value matched LeafNode.Value. This prevented WriteValueToSource()
from being called, so the validation error was never cleared.
Add an ErrorType check to the condition in WriteTargetValueToSource() so
that the write-through still happens when there is a pending validation
error, matching the pattern already used in WriteValueToSource() at
line 310.
Fixes#20534https://claude.ai/code/session_01LTinLuE1bsNKaLPuDSNuuw
* Add test for validation error clearing when reverting to same valid value
Reproduces the scenario from #20534 where a TwoWay binding with a
setter that throws on invalid values fails to clear the validation error
when the target is set back to the same value that was last successfully
written to the source.
https://claude.ai/code/session_01LTinLuE1bsNKaLPuDSNuuw
* Remove redundant checks in WriteTargetValueToSource()
Evan