Add [Obsolete] attribute to ReaderOptions.DefaultBufferSize for backward compatibility

[Copilot]

PR #1165 removed the public constant ReaderOptions.DefaultBufferSize, causing compilation errors for consumers referencing it. This PR restores the constant with deprecation guidance.

Changes

• Restored ReaderOptions.DefaultBufferSize with original value (0x10000 = 65536 bytes) and [Obsolete] attribute directing users to Constants.BufferSize
• Preserved new default behavior where BufferSize property defaults to Constants.BufferSize (81920 bytes, matching .NET's Stream.CopyTo)
• Added documentation explaining the value difference and backward compatibility preservation

Migration Path

Existing code compiles with warning instead of error:

// Old code - now produces CS0618 warning but still compiles
var bufferSize = ReaderOptions.DefaultBufferSize; // 65536

// New code - use this going forward
var bufferSize = Constants.BufferSize; // 81920

New ReaderOptions instances automatically use the improved 81920-byte default while existing constant references remain functional.

---

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