Make ZipArchiveEntry.CompressionMethod public

[iremyux]

This PR exposes the compression method of ZIP archive entries through a new public ZipCompressionMethod enum and a public CompressionMethod property on ZipArchiveEntry.

• Only supported compression methods are included.
• Enum values (0, 8, 9) correspond to the compression method values defined in the ZIP File Format Specification (APPNOTE.TXT section 4.4.5).
• Error handling for unsupported compression methods now uses a single generic message SR.UnsupportedCompression instead of method-specific messages for BZip2/LZMA.

Fixes #95909

[Copilot]

Pull request overview

This PR exposes the compression method of ZIP archive entries as a public API by introducing a new ZipCompressionMethod enum and a public CompressionMethod property on ZipArchiveEntry. The change replaces the internal CompressionMethodValues enum throughout the codebase and simplifies error handling for unsupported compression methods.

• Adds public ZipCompressionMethod enum with values for Stored (0), Deflate (8), and Deflate64 (9)
• Exposes CompressionMethod property on ZipArchiveEntry as a public getter
• Consolidates error messages for unsupported compression methods to use a single generic message

Reviewed changes

Copilot reviewed 7 out of 7 changed files in this pull request and generated 2 comments.

Show a summary per file

File	Description
src/libraries/System.IO.Compression/src/System/IO/Compression/ZipCompressionMethod.cs	New public enum defining supported ZIP compression methods (Stored, Deflate, Deflate64) with values matching ZIP specification
src/libraries/System.IO.Compression/src/System/IO/Compression/ZipArchiveEntry.cs	Replaces internal CompressionMethodValues with public ZipCompressionMethod; makes CompressionMethod property public; removes old internal enum; simplifies error handling
src/libraries/System.IO.Compression/src/System/IO/Compression/ZipArchiveEntry.Async.cs	Updates async methods to use ZipCompressionMethod instead of CompressionMethodValues
src/libraries/System.IO.Compression/src/System/IO/Compression/DeflateManaged/DeflateManagedStream.cs	Updates constructor parameter and assertions to use ZipCompressionMethod
src/libraries/System.IO.Compression/src/System.IO.Compression.csproj	Adds new ZipCompressionMethod.cs file to compilation
src/libraries/System.IO.Compression/ref/System.IO.Compression.cs	Adds public API surface for ZipCompressionMethod enum and CompressionMethod property
src/libraries/System.IO.Compression/tests/ZipArchive/zip_ReadTests.cs	Updates existing test to use ZipCompressionMethod; adds three new tests for Deflate, Stored, and empty file compression methods

[rzikm]

LGTM