Adds platform-aware package installation for #1963, including RID/TFM filtering, dependency selection, and content merging.

Changes:

• Adds runtime identifier and target framework filtering.
• Adds dependency-group parsing and merge support.
• Expands platform tests and CI authentication handling.

Suppressed comments (9)

src/code/PSResourceInfo.cs:744

• GetString() throws for a dependency group whose targetFramework is JSON null, and the outer conversion catch then rejects the entire package metadata. Treat null/non-string target frameworks as the existing AnyFramework fallback instead of calling GetString() unconditionally.

                                string tfmString = tfmElement.GetString();
                                if (!string.IsNullOrWhiteSpace(tfmString))
                                {
                                    NuGetFramework parsed = NuGetFramework.Parse(tfmString);

src/code/RuntimeIdentifierHelper.cs:178

• The explicit-target overload has the same reversed compatibility test, so -RuntimeIdentifier linux-x64 will include linux-musl-x64 assets even though the target is glibc-based. Only accept candidate RIDs found in the target RID's fallback chain; a candidate's descendants are not valid fallbacks for the target.

            // Check if the target is in the candidate RID's compatibility chain
            var candidateCompatibleRids = BuildCompatibleRidList(candidateRid);
            foreach (var compatibleRid in candidateCompatibleRids)
            {
                if (string.Equals(targetRid, compatibleRid, StringComparison.OrdinalIgnoreCase))

src/code/RuntimeIdentifierHelper.cs:340

• The musl RID graph is missing the generic linux-musl fallback. A package containing only runtimes/linux-musl/... is therefore not selected for linux-musl-x64, despite being the generic musl asset; add that node to the chain and cover it with a test.

                    compatibleRids.Add($"linux-{arch}");
                    compatibleRids.Add("linux");
                    compatibleRids.Add("unix");
                    compatibleRids.Add("any");

src/code/RuntimeIdentifierHelper.cs:405

• The unknown-OS fallback returns unix-{arch}, but this branch only adds any; consequently a package with a generic unix RID is excluded on that supported fallback platform. Include unix in the compatibility chain before any.

            else if (primaryRid.StartsWith("unix", StringComparison.OrdinalIgnoreCase))
            {
                // Generic Unix compatibility chain
                compatibleRids.Add("any");

src/code/RuntimePackageHelper.cs:57

• Requiring a hyphen excludes valid generic RIDs such as win, linux, osx, and unix from root-level filtering. A package containing win/native.dll is therefore treated as ordinary content and can be installed on non-Windows hosts, even though BuildCompatibleRidList models these RIDs. Include generic RIDs in the root-level detector.

            if (string.IsNullOrEmpty(folderName) || !folderName.Contains("-"))
            {
                return false;

src/code/RuntimePackageHelper.cs:64

• StartsWith alone accepts non-RIDs such as windows-x64 and linuxfoo-x64 because their final segment is a known architecture. On a nonmatching host those ordinary package folders are then removed as runtime assets; require a valid OS/RID token boundary or use a real RID parser.

            // Must start with a known OS prefix
            bool startsWithKnownOs = false;
            foreach (string prefix in s_knownOsPrefixes)
            {
                if (folderName.StartsWith(prefix, StringComparison.OrdinalIgnoreCase))

test/PSResourceInfo.Tests.ps1:67

• This test only supplies an empty dependency group, so it never exercises the newly added TFM selection or verifies that dependencies and version ranges from the selected group are retained. Add a JSON fixture with at least two framework groups and assert the dependency selected for the current host.

    It "Handles an empty dependencyGroups array" {
        $resourceInfo = [Microsoft.PowerShell.PSResourceGet.UtilClasses.TestHooks]::ConvertFromJson(
            '{"version":"1.0.0","id":"TestPackage","authors":"Test Author","dependencyGroups":[]}',
            $repository)

test/PlatformFilteringTests/PlatformAwareInstall.Tests.ps1:236

• Because the assertions are guarded by if (Test-Path $libDir), a regression that extracts no lib assets makes this test pass without checking any TFM. Assert that $libDir exists before enumerating it so the test actually verifies framework selection.

            if (Test-Path $libDir) {
                $installedTfmFolders = @((Get-ChildItem $libDir -Directory).Name)
                # Should have exactly 1 TFM folder (the best match)
                $installedTfmFolders.Count | Should -Be 1

test/PlatformFilteringTests/RuntimeIdentifierHelper.Tests.ps1:167

• This expectation encodes the reverse RID relationship that the production filter uses incorrectly: a generic win-x64 asset is compatible with a win10-x64 target, but a win10-x64 package asset is not a fallback for a win-x64 host. Test the target-aware overload in the valid direction so the test does not require the unsafe behavior.

            if ($currentRid -eq 'win-x64') {
                # win10-x64 package folder should be compatible on a win-x64 machine
                $InternalHooks::IsCompatibleRid('win10-x64') | Should -BeTrue
            }

• Files reviewed: 13/13 changed files
• Comments generated: 9
• Review effort level: Lite