Fixes #5639. Harden TextView text synchronization

[YourRobotOverlord]

Summary

Hardens TextView.Text synchronization when post-change notification throws or performs a reentrant polymorphic assignment.

• Fixes TextView new-Text setter: guard flag not cleared in finally — throwing TextChanged subscriber permanently breaks polymorphic Text sync #5639

Changes

• Finalize draw and history state before raising TextChanged.
• Reset the synchronization guard in finally after subscriber failures.
• Consume the guard before subscriber dispatch so reentrant View.Text assignments update the internal model.
• Add regression tests for exception recovery, history consistency, and reentrant polymorphic synchronization.

Testing

• dotnet build --configuration Debug --no-restore — passed with 0 errors and 3 pre-existing XML documentation warnings.
• Text CWP tests — 27 passed.
• TextView tests — 430 passed, 4 skipped.
• Non-parallel tests — 72 passed, 2 skipped.
• Full parallel suite — 17,585 passed, 17 skipped, and 4 unrelated ANSI color-encoding expectation failures in SchemeColorNoneDerivationTests and ViewDrawingClippingTests.

To pull down this PR locally:

git remote add copilot https://github.com/YourRobotOverlord/Terminal.Gui.git
git fetch copilot fix/5639-textview-setter-guard
git checkout copilot/fix/5639-textview-setter-guard

[Copilot]

Pull request overview

Hardens TextView.Text synchronization and recovery by ensuring internal state updates complete before TextChanged dispatch, and by making the synchronization guard resilient to exceptions and reentrant polymorphic View.Text assignments (fixing #5639).

Changes:

• Reworks TextView.Text setter / OnTextChanged guard logic to clear the guard via finally and allow reentrant polymorphic sets to resync the internal model.
• Finalizes draw + history baseline state before raising TextChanged so exceptions don’t leave TextView inconsistent.
• Adds regression tests covering throwing TextChanged subscribers, history baseline consistency, and reentrant polymorphic synchronization.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 2 comments.

File	Description
Terminal.Gui/Views/TextInput/TextView/TextView.Text.cs	Makes TextView text sync guard exception-safe and reentrancy-safe while preserving CWP behavior.
Tests/UnitTestsParallelizable/ViewBase/TextCwpTests.cs	Adds regression coverage for exception recovery, history baseline consistency, and reentrant polymorphic syncing.

Suppressed comments (2)

Tests/UnitTestsParallelizable/ViewBase/TextCwpTests.cs:195

• AI-generated test marker comment does not follow the required repository format. .claude/rules/testing-patterns.md specifies the marker must be either // Claude - <model> or // CoPilot - <model> (both accepted).

    // Codex - GPT-5.6

Tests/UnitTestsParallelizable/ViewBase/TextCwpTests.cs:208

• AI-generated test marker comment does not follow the required repository format. .claude/rules/testing-patterns.md specifies the marker must be either // Claude - <model> or // CoPilot - <model> (both accepted).

    // Codex - GPT-5.6

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