Allow Flush as no-op on read-only streams

[Copilot]

Read-only streams (SubstreamReader, NestedStream, ReadOnlySequenceStream) were throwing NotSupportedException on Flush operations. Per dotnet/runtime#105592 and #110899, they should allow Flush as a no-op.

Changes

• Stream implementations: Modified Flush() and FlushAsync() to check disposal state and return immediately instead of throwing NotSupportedException
• Tests: Updated to verify Flush succeeds on active streams while still throwing ObjectDisposedException when disposed

Example

Before:

var stream = underlyingStream.ReadSubstream();
stream.Flush(); // Throws NotSupportedException

After:

var stream = underlyingStream.ReadSubstream();
stream.Flush(); // No-op, succeeds

Implementation uses Task.CompletedTask (.NET 5+) or Task.FromResult(0) for async variants.

Original prompt

---

This section details on the original issue you should resolve

<issue_title>Read-Only streams should do nothing in Flush</issue_title>
<issue_description>Instead of throwing, streams should allow flush as a no-op.

See also dotnet/runtime#105592 and dotnet/runtime#110899.

Currently, SubstreamReader, NestedStream, ReadOnlySequenceStream explicitly throw NotSupportedException instead</issue_description>

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

• Fixes Read-Only streams should do nothing in Flush #917

---

💬 We'd love your input! Share your thoughts on Copilot coding agent in our 2 minute survey.