feat(roslyn_ls): support override completion and refresh diagnostics #4050

Problem:
- Diagnostics were only refreshed for the current buffer, not all buffers
  managed by the client. In multi-file C# projects, this left diagnostics
  stale unless the user switched buffers.
- Roslyn did not handle override method completion correctly.

Solution:
- Added `roslyn.client.completionComplexEdit` handler to apply edits for
  override completions.
- Added `refresh_diagnostics` function, called on project initialization
  and on `BufWritePost` / `InsertLeave` events, to proactively pull
  diagnostics for all client buffers.

Rationale:
- Roslyn LSP primarily uses pull-based diagnostics. Neovim currently
  only triggers diagnostic pulls on text changes in the active buffer.
  This makes `debounce_text_changes` insufficient for Roslyn, as changes
  in one file can affect diagnostics across the entire solution.
- The autocmds act as a server-specific workaround to mimic an
  “on-save/on-idle” pull model.

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