C#: fix InstallRecipes echoing wildcard versions in response.Version

[natedanner]

Summary

• Fixed: the InstallRecipes RPC was returning the literal version-constraint string (*, *-*, etc.) as InstallRecipesResponse.Version instead of the resolved concrete SemVer. Downstream consumers (moderne-cli recipes-v5.csv, moderne-saas GraphQL) surfaced these as the "installed version" and had to work around it.
• Root cause: dotnet add package <pkg> --version * preserves the wildcard verbatim in the csproj's <PackageReference>. The old ResolveVersionFromCsproj read from that attribute, so wildcards flowed through unchanged. Verified against real dotnet output (Version="*" is written literally).
• Fix: new public helper MSBuildProjectHelper.GetResolvedPackageVersion(projectDir, packageName) reads obj/project.assets.json (NuGet's per-TFM resolution map written by restore) and returns the resolved version of a direct dependency. Reads from targets to match the convention already used by ReadResolvedPackages in the same file. Throws InvalidOperationException with specific diagnostics for: missing project.assets.json, package not a direct dep, or no matching resolved entry.
• InstallRecipes call site (RewriteRpcServer.cs) now delegates to the helper; the old private ResolveVersionFromCsproj is removed along with the unused System.Xml.Linq import.
• Regression coverage in OpenRewrite.Tests/Rpc/InstallVersionResolverTests.cs: theory rows for * and *-* against concrete-resolved fixtures, plus facts for case-insensitive package match, unknown-package throw, and missing-assets-json throw.

Test plan

• dotnet test --filter InstallVersionResolverTests — 5/5 pass in ~10 ms.
• ./gradlew :rewrite-csharp:csharpTest — 1936/1936 pass.
• Mutation test: replacing the targets lookup with project.frameworks.<tfm>.dependencies.<pkg>.version (the original bug shape) causes the two wildcard rows + case-insensitive case to fail (3/5 failures). Confirms the tests actually cover the regression.
• Verified against real dotnet add package Newtonsoft.Json --version "*" output: csproj receives Version="*"; obj/project.assets.json contains targets.<tfm>."Newtonsoft.Json/13.0.4" and libraries."Newtonsoft.Json/13.0.4"; helper returns 13.0.4.

Out of scope

• Pre-existing _recipesProjectDir caching across InstallRecipes calls (no per-call cleanup, no concurrency control). Not introduced by this PR.
• moderne-cli and moderne-saas workarounds that scan ~/.nuget/packages/ or post-process the version string; those become dead code once this fix ships.

[macsux]

/lgtm