Add Roslyn LSP entry-point query for file-based programs and refresh discovery

[Copilot]

Roslyn already had file-based-program entry-point heuristics, but the client was adding its own copy. This change exposes Roslyn’s decision over LSP and fixes the known discovery gaps around top-level statements, workspace-folder updates, and newly created .cs files.

• New LSP surface

  • Adds workspace/_roslyn_isFileBasedProgramEntryPoint so clients can ask Roslyn whether a file should be treated as an FBA entry point under the current workspace state.
  • Moves the heuristic behind a reusable server-side API instead of duplicating it in the client.

  {
    "method": "workspace/_roslyn_isFileBasedProgramEntryPoint",
    "params": {
      "textDocument": {
        "uri": "file:///workspace/App.cs"
      }
    }
  }

• Shared entry-point heuristic

  • Factors entry-point detection into shared logic used by both open-file classification and background discovery.
  • Recognizes:
    • #! files as explicit entry points
    • #: + top-level statements as explicit entry points
    • top-level statements without directives as ambiguous entry points

• Discovery correctness

  • Updates automatic discovery to use the same heuristic as classification, instead of only looking for #! / #: markers.
  • This allows top-level-statement files to participate in discovery when the ambiguous-file path is enabled.

• Workspace change handling

  • Adds workspace/didChangeWorkspaceFolders handling.
  • Updates InitializeManager and CsprojInConeChecker so workspace-folder state is live rather than captured once at initialization.
  • Re-runs discovery when folders are added or removed.

• File watching

  • Adds workspace-folder watches for .cs and .csproj.
  • Re-runs discovery when relevant files are created or changed, so new candidate entry points can be loaded without restarting the server.

• Unit test coverage

  • Adds coverage for:
    • top-level-statements-only discovery
    • rescanning after workspace-folder changes
    • discovering new files from watched-file notifications
    • the new LSP entry-point request

Microsoft Reviewers: Open in CodeFlow

[RikkiGibson]

We chose to require the leading #! to make a file-based app auto-discoverable, in part because doing this parse on all the C# files in a workspace folder is taxing on memory and cpu.

If we want to move forward with this, we should def be measuring and comparing the perf very carefully.

To be honest I am also a little hesitant about CreateTokenParser based solution for auto-discovery as creating SourceText requires reading the whole file content from disk (checksum is produced eagerly). This extra I/O is hard to notice on an nvme drive when auto-discovery is the only thing running, but, in a real editor startup scenario, there's a lot of contention, and being able to read less (<= ~4KB, say), does help a bit, IIRC.

[mwiemer-microsoft]

This worked on microsoft/vscode-dotnettools#2369 but introduces too many changes at once. We're going to improve this heuristic with a series of PRs for reviewability.