TextStore: initialize callback state before RegisterTextStore

[etvorun]

Summary

This fixes a TextStore attach-time reentrancy window where TSF callbacks can arrive during TextStore.OnAttach() before attach-time state is fully safe.

What changed

In TextStore.OnAttach():

• Initialize callback-facing state (_textservicesproperty) before RegisterTextStore.
• Subscribe TextContainer.Change before RegisterTextStore so registration-time edits update _netCharCount immediately.

Why

RegisterTextStore can synchronously re-enter WPF with third-party IMEs/TIPs. If attach-time edits happen before text-change tracking is hooked, _netCharCount can drift from TextContainer.IMECharCount, causing later consistency failures.

Validation

• Reproduced attach/focus-time callback reentrancy with a custom native TIP test DLL.
• Verified _netCharCount remains synchronized in this path.

Fixes #11456

Microsoft Reviewers: Open in CodeFlow

[Copilot]

Pull request overview

This PR fixes a critical reentrancy bug in the WPF TextStore's TSF (Text Services Framework) integration where third-party IMEs can trigger callbacks during RegisterTextStore, before all initialization state is ready. The fix reorders initialization in OnAttach() to ensure callback-facing state is fully initialized before TSF registration.

Changes:

• Initialize _textservicesproperty before RegisterTextStore to prevent null dereference in TSF callbacks
• Subscribe to TextContainer.Change before RegisterTextStore to keep _netCharCount synchronized during registration-time edits
• Add detailed comments explaining the reentrancy scenario and specific IME that exhibits this behavior

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[miloush]

LGTM

[h3xds1nz]

Should be all these #9829 #3315 #7304 #10083 #9805

[etvorun]

Should be all these #9829 #3315 #7304 #10083 #9805

#7304 does not look like the same issue