Add NuGet symbol package support and SourceLink for debugging

[ihorleleka]

Motivation

• Enable consumers to debug the library when installed from NuGet by shipping portable PDBs and a symbol package.
• Improve source stepping by publishing repository metadata and embedding untracked sources so debuggers can resolve source files.

Description

• Update Code/IL.InMemoryCacheProvider.csproj to enable symbol packaging by adding IncludeSymbols, SymbolPackageFormat set to snupkg, and DebugType set to portable.
• Add PublishRepositoryUrl and EmbedUntrackedSources to include repository metadata and untracked sources in the package.
• Add Microsoft.SourceLink.GitHub as a PackageReference to enable Source Link support for stepping into package source.

Testing

• Attempted dotnet test IL.InMemoryCacheProvider.slnx, which could not run because the dotnet SDK is not installed in this environment (failed).
• Attempted dotnet pack Code/IL.InMemoryCacheProvider.csproj -c Release -o artifacts, which could not be executed in this environment and did not complete (failed).

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Codex Task