HostedFileDownloadStream did not consistently advertise or enforce its read-only contract at the base type. This change makes the abstraction explicitly non-writable so all implementations inherit the same behavior by default.

• Base stream contract

  • Override CanWrite in HostedFileDownloadStream to always return false
  • Make all write-entry points throw NotSupportedException at the base class:
    • SetLength
    • BeginWrite / EndWrite
    • WriteByte
    • Write(byte[], ...)
    • WriteAsync(byte[], ...)
    • Write(ReadOnlySpan<byte>)
    • WriteAsync(ReadOnlyMemory<byte>)

• Implementation cleanup

  • Remove duplicated write-path overrides from OpenAIFileDownloadStream
  • Let the concrete implementation inherit the shared read-only behavior from the base type

• Coverage

  • Add focused tests asserting:
    • CanWrite == false
    • write-related APIs throw NotSupportedException

public override bool CanWrite => false;

public override void Write(byte[] buffer, int offset, int count) =>
    throw new NotSupportedException();

public override Task WriteAsync(byte[] buffer, int offset, int count, CancellationToken cancellationToken) =>
    throw new NotSupportedException();

Microsoft Reviewers: Open in CodeFlow