Fix RichTextBox sink event loss during disposal

[vonhoff]

The RichTextBox sink could display only the first of several rapidly emitted log events when the logger was immediately closed, especially when combined with the File sink.

This change drains the final buffered snapshot before stopping the sink’s processing task, preserving events emitted before disposal. It also adds regression coverage, and bumps the package version to 3.2.1

[Copilot]

🟡 Changes recommended

A critical disposal hang risk and dependency compatibility issue remain unresolved.

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Pull request overview

Fixes RichTextBox sink event loss during disposal by draining buffered events, adding regression coverage, and updating targets and package metadata.

Changes:

• Flushes the final buffered snapshot during shutdown.
• Adds sink lifecycle regression coverage.
• Updates targets, dependencies, and package version.

File summaries

File	Summary
Serilog.Sinks.RichTextBox.WinForms.Colored/Sinks/RichTextBoxForms/RichTextBoxSink.cs	Adds final flushing; cancellation handling may allow disposal to hang.
Serilog.Sinks.RichTextBox.WinForms.Colored/Serilog.Sinks.RichTextBox.WinForms.Colored.csproj	Updates package metadata; raises the Serilog minimum to 4.4.0 without documented justification.
Serilog.Sinks.RichTextBox.WinForms.Colored.Test/Serilog.Sinks.RichTextBox.WinForms.Colored.Test.csproj	Updates test tooling and dependencies.
Serilog.Sinks.RichTextBox.WinForms.Colored.Test/Integration/SinkLifecycleTests.cs	Adds disposal event-draining coverage.
Demo/Demo.csproj	Updates demo dependencies.

Review details

Suppressed comments (1)

Serilog.Sinks.RichTextBox.WinForms.Colored/Sinks/RichTextBoxForms/RichTextBoxSink.cs:154

• This call still does not synchronously apply the final snapshot: ProcessMessages runs on Task.Run, and SetRtf uses BeginInvoke when InvokeRequired is true, so this returns after only queuing the UI delegate. Dispose() waits for _processingTask but not that delegate; if the caller disposes the control immediately afterward, the handle can be destroyed before the callback runs and the final events are still lost. Shutdown needs to acknowledge the final UI callback without blocking the UI thread, and this path needs a test that disposes the control immediately.

                _richTextBox.SetRtf(builder.Rtf, _options.AutoScroll, stopping ? CancellationToken.None : token);

• Files reviewed: 5/5 changed files
• Comments generated: 2
• Review effort level: Lite

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