Previously, when an LSP server was configured but not installed, the error
message said "No LSP server configured" which was misleading. Now the
error message distinguishes between:
1. Server not configured at all
2. Server configured but not installed (with installation hints)
The new error messages include:
- Clear indication of whether server is configured vs installed
- Installation commands for each built-in server
- Supported file extensions
- Configuration examples for custom servers
Fixes#304
Co-authored-by: sisyphus-dev-ai <sisyphus-dev-ai@users.noreply.github.com>