[Repo Assist] fix: improve test infrastructure resilience (flaky diagnostics + cleanup failures)

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Summary

Fixes two pre-existing flaky test infrastructure issues that cause intermittent CI failures.

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Fix 1: waitForLatestDiagnostics throws on empty diagnostics buffer

File: test/FsAutoComplete.Tests.Lsp/Utils/Server.fs

Problem: waitForLatestDiagnostics called Seq.last on the buffered diagnostic notifications. When the language server produces no diagnostic notifications (e.g., for error-free code), Observable.bufferSpan completes with an empty buffer and Seq.last throws:

The input sequence was empty. (Parameter 'source')
  at Utils.Server.DocumentModule.waitForLatestDiagnostics@301-9
```

This caused the `InlayHint.voption.show: for custom function` test to fail intermittently on ubuntu-latest/net9.0.

**Fix:** Replace `Seq.last` with `Seq.tryLast |> Option.defaultValue [||]` to return an empty diagnostics array when no notifications are received.

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### Fix 2: `dotnetCleanup` throws when directory deletion fails

**File:** `test/FsAutoComplete.Tests.Lsp/Helpers.fs`

**Problem:** `dotnetCleanup` called `Directory.Delete(path, true)` directly. On macOS with .NET 10, this throws `"Directory not empty"` because background SDK processes (hot reload daemon, etc.) still hold file locks on some artifacts:

```
System.IO.FileSystem.RemoveDirectoryRecursive(String fullPath)
at Helpers.dotnetCleanup(String baseDir)

This caused 15 test failures in Go to definition tests.GoTo Tests on the macos-15 / net10.0 / TransparentCompiler CI matrix (pre-existing failures visible on main branch, run #2908).

Fix: Wrap the deletion in a try-catch, retry once after 200ms (for transient locks), and then silently continue if cleanup still fails. Stale build artifacts don't prevent tests from running correctly.

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Test Status

Build: ✅ dotnet build test/FsAutoComplete.Tests.Lsp -f net8.0 — 0 errors, 0 new warnings.

Formatting: ✅ dotnet fantomas --check — both files pass.

These are test-infrastructure-only changes (no production code modified). The fixes remove sources of intermittent CI failure without changing any test assertions or test logic.

Closes #700 (partially — the dotnetCleanup fix addresses one source of test flakiness)

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