Fix encoding missed overridden properties

[tarekgh]

Fixes #113059

When we introduced the codepage encodings library, we missed supporting the encoding properties WindowsCodePage, IsBrowserDisplay, IsBrowserSave, IsMailNewsDisplay, and IsMailNewsSave. The current behavior is these properties will throw NotSupportedException.
Some users hit this issue when migrating from .NET Framework to .NET.

PR Overview

This PR fixes missing support for several encoding properties (WindowsCodePage, IsBrowserDisplay, IsBrowserSave, IsMailNewsDisplay, and IsMailNewsSave) that previously threw NotSupportedException.

• Adds new tests and MemberData to verify the proper mapping of code pages to Windows code pages and flags.
• Implements overrides in EncodingNLS and updates EncodingTable and EncodingTable.Data to provide the correct MIME flags and family code pages using caching.

Reviewed Changes

File	Description
src/libraries/System.Text.Encoding.CodePages/tests/EncodingCodePages.cs	Added tests for WindowsCodePage and flag properties and introduced new MemberData with the code page mapping.
src/libraries/System.Text.Encoding.CodePages/src/System/Text/EncodingNLS.cs	Implemented new overrides for encoding properties using the cached code page item lookup.
src/libraries/System.Text.Encoding.CodePages/src/System/Text/EncodingTable.Data.cs	Defined new MIME flag constants and populated MappedFlags with the appropriate values.
src/libraries/System.Text.Encoding.CodePages/src/System/Text/EncodingTable.cs	Added a cache and implemented GetCodePageItem to return family code page and flags.

Copilot reviewed 5 out of 5 changed files in this pull request and generated no comments.

Comments suppressed due to low confidence (1)

src/libraries/System.Text.Encoding.CodePages/tests/EncodingCodePages.cs:782

• Rather than silently returning when the encoding is null, consider asserting that encoding is not null so that unexpected missing encodings are flagged in the test results.

if (encoding is null) // Not supported encoded like utf cases

[carlossanlop]

Do you need to re-run the ref source update for this assembly? https://github.com/dotnet/runtime/blob/main/docs/coding-guidelines/updating-ref-source.md

Edit: Nevermind, internal class.

[carlossanlop]

LGTM. Feel free to consider or ignore my suggestion for the returned tuple values.

[carlossanlop]

@MichaelSimons @ViktorHofer this package reference is causing the error in CI about prebuilts.

I tried adding the package to source-build-reference-packages but I get this error:

~/repos/source-build-reference-packages $ ./generate.sh --package System.ValueTuple,4.5.0
...
Build succeeded in 4.6s
Restore complete (1.9s)
  PackageSourceGeneratorTask succeeded (2.9s) → artifacts/bin/PackageSourceGeneratorTask/Debug/net10.0/PackageSourceGeneratorTask.dll
  PackageSourceGenerator succeeded (0.4s) → Package source generation skipped for System.ValueTuple (v4.5.0) as it doesn't contain any compile items.

What options do we have for such a case where no compile items are found in a published package, but adding such package as reference causes prebuilt errors?

[ViktorHofer]

Replied offline already. The System.ValueTuple package is a netfx support package that doesn't need to be referenced on NET Standard.

Make sure that the package ref is conditioned to TargetFrameworkIdentifier == NETFramework.

[tarekgh]

/ba-g the failures are infrastructure and looks temporary.