Add Encoding.WithoutPreamble() extension method

[Copilot]

Adds a general-purpose Encoding.WithoutPreamble() extension that produces a derived encoding with an empty preamble, applicable to any Encoding — not just UTF-8.

Changes

• NoPreambleEncoding (file-local) — sealed wrapper that delegates all abstract encode/decode methods and identity properties (CodePage, EncodingName, WebName, etc.) to the inner encoding, overriding only GetPreamble() to return Array.Empty<byte>()
• WithoutPreamble() extension — returns this if the encoding already has no preamble; otherwise wraps in NoPreambleEncoding
• Utf8WithoutBom updated — now implemented via Encoding.UTF8.WithoutPreamble() instead of new UTF8Encoding(false)

Usage

// Any encoding — strip the preamble
var encoding = new UTF8Encoding(encoderShouldEmitUTF8Identifier: true);
var noBom = encoding.WithoutPreamble();
noBom.GetPreamble(); // []

// Already preamble-free — same instance returned
var ascii = Encoding.ASCII.WithoutPreamble(); // === Encoding.ASCII

[Copilot]

Pull request overview

Adds a general-purpose way to strip BOM/preamble from any Encoding by introducing an Encoding.WithoutPreamble() extension and refactoring Encoding.Utf8WithoutBom to use it.

Changes:

• Added a file-local NoPreambleEncoding wrapper that overrides GetPreamble() to return an empty preamble while delegating encoding/decoding operations to an inner Encoding.
• Added Encoding.WithoutPreamble() extension that returns the original encoding when it already has no preamble, otherwise wraps it.
• Updated Utf8WithoutBom to use Encoding.UTF8.WithoutPreamble() and added tests for the new behavior.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 2 comments.

File	Description
PowerKit/Extensions/EncodingExtensions.cs	Implements NoPreambleEncoding, adds WithoutPreamble(), and updates Utf8WithoutBom to use the new extension.
PowerKit.Tests/EncodingExtensionsTests.cs	Adds unit tests covering WithoutPreamble() behavior and instance-return semantics.

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[Tyrrrz]

@copilot build errors

[Copilot]

@copilot build errors

Fixed in 7c11b70. Two net35 build errors:

1. Encoding(int, EncoderFallback, DecoderFallback) — this 3-arg constructor was introduced in .NET 4.0 and doesn't exist in net35. Switched back to the default parameterless constructor; the cloned _inner already carries the source encoding's fallbacks so all encode/decode operations are correct.
2. [] (collection expression) — not available in older C# language versions targeting net35. Kept as new byte[0].