Add ZstandardDecompressionOptions for symmetric decoder configuration

[Copilot]

Closes #128456

ZstandardDecoder/ZstandardStream had no options-based constructor analogous to ZstandardCompressionOptions, making it impossible for callers (e.g. ASP.NET Core middleware) to cleanly configure and tie decoder lifetime to the stream.

Changes

• ZstandardDecompressionOptions — new sealed class with MaxWindowLog (int, validated) and Dictionary (ZstandardDictionary?) properties, mirroring the compression counterpart
• ZstandardDecoder(ZstandardDecompressionOptions) — new constructor applying window log and dictionary from options
• ZstandardStream(Stream, ZstandardDecompressionOptions, bool leaveOpen = false) — stream creates and owns the decoder (disposed on stream dispose, unlike the ZstandardDecoder-accepting overload which resets but does not dispose)
• Platform-not-supported stubs for browser/wasi, reference assembly updated

// Stream now owns decoder lifetime — no manual disposal needed
return new ZstandardStream(stream, new ZstandardDecompressionOptions
{
    Dictionary = _dictionary,
    MaxWindowLog = 24,
}, leaveOpen: true);

Testing

• 351 Zstandard tests pass (0 failures)
• New ZstandardDecompressionOptionsTests: property range validation, null/default values
• New decoder constructor tests: null arg, default, window log, dictionary round-trip
• New stream constructor tests: null check, basic decompression, ownership/disposal, dictionary round-trip

[dotnet-policy-service]

Tagging subscribers to this area: @karelz, @dotnet/area-system-io-compression
See info in area-owners.md if you want to be subscribed.

[Copilot]

Pull request overview

This PR expands the System.IO.Compression Zstandard surface area by introducing decompression options and new overloads so callers can configure decoding (dictionary + max window log) via an options object, including a new ZstandardStream decompression constructor.

Changes:

• Add new public ZstandardDecompressionOptions type (dictionary + MaxWindowLog) and wire it into ZstandardDecoder and ZstandardStream via new overloads.
• Update browser/wasi platform-not-supported stubs and the reference assembly to include the new public APIs.
• Add unit tests covering options validation and basic options-based decoding/stream usage.

Reviewed changes

Copilot reviewed 9 out of 9 changed files in this pull request and generated 4 comments.

Show a summary per file

File	Description
src/libraries/System.IO.Compression/src/System/IO/Compression/Zstandard/ZstandardDecompressionOptions.cs	New public options type for configuring Zstandard decompression.
src/libraries/System.IO.Compression/src/System/IO/Compression/Zstandard/ZstandardDecoder.cs	New ZstandardDecoder(ZstandardDecompressionOptions) constructor that applies options.
src/libraries/System.IO.Compression/src/System/IO/Compression/Zstandard/ZstandardStream.Decompress.cs	New ZstandardStream(Stream, ZstandardDecompressionOptions, bool) decompression constructor creating a decoder from options.
src/libraries/System.IO.Compression/src/System/IO/Compression/Zstandard/Zstandard.PlatformNotSupported.cs	Add stubs for the new options type and new overloads on browser/wasi.
src/libraries/System.IO.Compression/src/System.IO.Compression.csproj	Include the new source file in the Zstandard compilation item group.
src/libraries/System.IO.Compression/ref/System.IO.Compression.cs	Update ref surface area to include the new public type and overloads.
src/libraries/System.IO.Compression/tests/Zstandard/ZstandardEncoderDecoderTests.cs	Add tests for the new decoder options constructor (null/default/windowlog/dictionary).
src/libraries/System.IO.Compression/tests/Zstandard/ZstandardCompressionOptionsTests.cs	Add tests for ZstandardDecompressionOptions property validation and defaults.
src/libraries/System.IO.Compression/tests/Zstandard/CompressionStreamUnitTests.Zstandard.cs	Add tests for the new options-based stream constructor (basic usage + null + dictionary round-trip).

[rzikm]

@copilot address code review comments

[Copilot]

@copilot address code review comments

Addressed all code review comments in commit 6bef7c9:

• OverloadResolutionPriority(-1): Added [OverloadResolutionPriority(-1)] to ZstandardDecoder(ZstandardDecompressionOptions) in the implementation, platform-not-supported stubs, and ref assembly to prevent new ZstandardDecoder(null) ambiguity.
• Ownership/disposal test: Added ZstandardStream_WithDecompressionOptions_DisposedStream_ThrowsObjectDisposedException verifying that reads after disposal throw ObjectDisposedException.
• Redundant zero test (ZstandardCompressionOptionsTests.cs:83): Removed the separate MaxWindowLog_SetToZero_Succeeds fact and folded 0 as an [InlineData] into MaxWindowLog_SetToValidRange_Succeeds.
• Below-minimum test coverage (ZstandardCompressionOptionsTests.cs:104): Added [InlineData(1)] (well below minimum of 10) alongside the existing boundary values.
• Duplicate decoder test (ZstandardEncoderDecoderTests.cs:603): Merged Default_Succeeds and WithMaxWindowLog_Succeeds into a single [Theory] with [InlineData(0)] and [InlineData(10)].

[Copilot]

Pull request overview

Copilot reviewed 9 out of 9 changed files in this pull request and generated 4 comments.

[rzikm]

@copilot address the code review comments

[alinpahontu2912]

Looks fine to me, but there is no test with leaveopen:false, would we need that too?

[rzikm]

would we need that too?

I think it's fine to not have them for absolutely all cases