Flaky test: FCS tooltip `#r` reference-assembly resolution is non-deterministic (Fsx.Bug5073, ResolveFromGAC)

**Description**

A couple of `#r` reference-assembly *tooltip* tests in `tests/FSharp.Compiler.Service.Tests/Tooltip/TooltipTests.Declarations.fs` intermittently produce an **empty tooltip** in the FCS test host, so the `assertReferenceTooltipContains` assertions flake:

- `Fsx.Bug5073` — `#r "System"`, expects the tooltip to contain `Reference Assemblies\Microsoft`
- `Fsx.HashR_QuickInfo.ResolveFromGAC` — `#r "CustomMarshalers"`, expects `.NETFramework` / `CustomMarshalers.dll`

The root cause looks like non-deterministic reference-assembly / GAC name resolution inside the FCS test host rather than a real product regression (the same `#r` resolves fine in VS/`dotnet fsi`).

**Current state**

- `ResolveFromGAC` is already skipped on `main` with `[]`.
- `Fsx.Bug5073` was briefly skipped in PR #20100 (the maestro Roslyn dependency bump) to keep that PR green, but the skip was reverted so the flake isn't hidden inside an unrelated dependency update. This issue tracks it instead.

**Repro steps**

1. Run `dotnet test tests/FSharp.Compiler.Service.Tests --filter "FullyQualifiedName~Fsx.Bug5073"` (Windows/Desktop; these are `FactForDESKTOP`).
2. Re-run several times — the tooltip is occasionally empty and the assertion fails.

**Expected behavior**

The reference tooltip for `#r "System"` / `#r "CustomMarshalers"` resolves deterministically and the tests pass reliably.

**Actual behavior**

Intermittently empty tooltip → `assertReferenceTooltipContains` fails.

**Suggested fix**

Make the test host's reference-assembly/GAC resolution deterministic (or harden the tooltip resolution / add a retry-until-ready in the test helper), then re-enable both tests.

**Known workarounds**

Skip the tests (undesirable — hides the flake).

Start in tests/FSharp.Compiler.Service.Tests/Tooltip/TooltipTests.Declarations.fs and run the documented dotnet test filter on Windows/Desktop, repeating it to reproduce the empty tooltip. Compare Fsx.Bug5073 and the skipped Fsx.HashR_QuickInfo.ResolveFromGAC while tracing reference-assembly/GAC resolution in the FCS test host. Done means both tests resolve their expected tooltip text deterministically and can be re-enabled.

Written by the indexing model from the issue text.