chore: drop redundant System.Collections.Immutable reference (NU1510)

[jeremydmiller]

Closes #583

What

Every restore emitted:

src/CodegenTests/CodegenTests.csproj : warning NU1510: PackageReference
System.Collections.Immutable will not be pruned. Consider removing this package
from your dependencies, as it is likely unnecessary.

CodegenTests genuinely uses the type — src/CodegenTests/VariableTests.cs:62 references ImmutableHashSet<string> — but System.Collections.Immutable has been in-box since .NET Core 3.x, and this project targets net9.0;net10.0. The type resolves from the framework either way, so the explicit reference adds nothing.

Beyond the one line the issue called for: CodegenTests was the only consumer, so the matching PackageVersion entry in Directory.Packages.props is removed too rather than left behind as a dangling central version.

Why it matters

This was the last remaining restore warning in the solution now that #579 cleared the MessagePack advisories (and #585 clears the build/ ones). dotnet restore jasperfx.slnx is now completely clean — which is the point: it makes future restore warnings actually visible instead of arriving in a pile of known noise.

Verification

$ dotnet restore jasperfx.slnx
  (no NU1510 — no warnings at all)

$ dotnet test src/CodegenTests --framework net9.0
  Passed! - Failed: 0, Passed: 419, Skipped: 0, Total: 419

419 matches the baseline recorded in #581. VariableTests.cs compiles unchanged on the framework-provided type.

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