Call FlushAsync before Dispose in DisposeAsync polyfills for Stream and TextWriter

[Copilot]

• Update PolyShim/NetCore30/Stream.cs – call FlushAsync before Dispose in the DisposeAsync fallback path (with try/finally to guarantee disposal)
• Update PolyShim/NetCore30/TextWriter.cs – call FlushAsync before Dispose in the DisposeAsync fallback path (with try/finally to guarantee disposal)
• Simplify PolyShim.Tests/NetCore30/StreamTests.cs – single focused DisposeAsync_Test using a BufferedStream with WriteAsync, verifying data is flushed before disposal
• Remove PolyShim.Tests/NetCore30/TextWriterTests.cs per review feedback

Original prompt

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This section details on the original issue you should resolve

<issue_title>DisposeAsync should call FlushAsync on types with asynchronous IO before Dispose</issue_title>
<issue_description>### Details

Currently, PolyShim implements DisposeAsync for several types by simply wrapping Dispose. However, for types where Dispose may synchronously finalize IO operations (such as streams or writers), DisposeAsync should call FlushAsync or an equivalent asynchronous method before Dispose. This ensures that any remaining data is flushed asynchronously, avoiding blocking synchronous operations in Dispose and providing more consistent async IO semantics.

Proposal:

• Update DisposeAsync implementations to invoke FlushAsync or similar methods where available, before calling Dispose.
• This change should apply to any types in PolyShim where FlushAsync is relevant (streams, writers, etc.).
• If a type does not support FlushAsync or an equivalent, the logic can remain unchanged.

This will enhance the correctness and performance of DisposeAsync in scenarios that involve asynchronous IO, aligning with expected .NET async patterns.

Checklist

• I have looked through existing issues to make sure that this feature has not been requested before
• I have provided a descriptive title for this issue
• I am aware that even valid feature requests may be rejected if they do not align with the project's goals
• I have sponsored this project</issue_description>

<agent_instructions>Also look for any other DisposeAsync() polyfills that may benefit from similar treatment</agent_instructions>

Comments on the Issue (you are @copilot in this section)

• Fixes DisposeAsync should call FlushAsync on types with asynchronous IO before Dispose #83

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[Tyrrrz]

@copilot

[Copilot]

Pull request overview

This PR updates the NetCore30 DisposeAsync polyfills for Stream and TextWriter to perform an async flush before synchronous disposal in the fallback path (when IAsyncDisposable isn’t available), and adds tests intended to validate the behavior.

Changes:

• Update Stream.DisposeAsync polyfill to await FlushAsync() and then Dispose() in finally.
• Update TextWriter.DisposeAsync polyfill similarly.
• Add new unit tests for DisposeAsync on Stream and TextWriter.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 4 comments.

File	Description
PolyShim/NetCore30/Stream.cs	Replace Task.Run(Dispose) fallback with FlushAsync + Dispose in finally.
PolyShim/NetCore30/TextWriter.cs	Replace Task.Run(Dispose) fallback with FlushAsync + Dispose in finally.
PolyShim.Tests/NetCore30/StreamTests.cs	Adds tests for DisposeAsync and “flushes data” scenario.
PolyShim.Tests/NetCore30/TextWriterTests.cs	Adds tests for DisposeAsync and “flushes data” scenario.

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 0.00%. Comparing base (0c7f42a) to head (d08724c).
⚠️ Report is 1 commits behind head on master.

Additional details and impacted files

@@     Coverage Diff      @@
##   master   #84   +/-   ##
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