[repo-assist] Fix #3603: paket pack reads Package-prefixed metadata from SDK-style csproj

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Closes #3603

Root cause

ProjectFile.GetTemplateMetadata() (used by paket pack to read NuGet metadata directly from .csproj files) only read the legacy plain MSBuild property names: ReleaseNotes, ProjectUrl, IconUrl, LicenseExpression, LicenseUrl, RequireLicenseAcceptance, Tags.

SDK-style (new-format) .csproj files conventionally set the Package-prefixed equivalents instead: PackageReleaseNotes, PackageProjectUrl, PackageIconUrl, PackageLicenseExpression, PackageLicenseUrl, PackageRequireLicenseAcceptance, PackageTags. As a result, paket pack silently produced packages missing release notes (and other metadata) for SDK-style projects using these properties.

This is also inconsistent with Paket's own Paket.Restore.targets, which already maps PackageReleaseNotes → ReleaseNotes when invoking PackTask.

Fix

Added propOrPackage/propMapOrPackage helpers in GetTemplateMetadata() that try the plain property name first, then fall back to the Package-prefixed name. This preserves existing behavior for legacy-style projects (plain name always wins if present) while fixing the gap for SDK-style projects.

Trade-offs

Only extended fields with an established Package-prefixed MSBuild convention (matching what Paket.Restore.targets already expects). Fields like Title, Owners, Summary, Readme, Language, Icon, Copyright, and repository info were left unchanged since they don't have an equivalent SDK convention.

Test Status

• Build: ✅ dotnet build succeeds with no new warnings/errors.
• Targeted tests: ✅ Added two new tests in tests/Paket.Tests/Packaging/PackageProcessSpecs.fs verifying the fallback and that plain properties still take precedence — both pass.
• Full PackagingProcess test module: 5/6 pass; the one pre-existing failure (Loading assembly metadata works, a version-string mismatch) is unrelated to this change and reproduces on master as well.

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