Abort rather than calling arduino-cli compile with a blank --fqbn.

The server calls arduino-cli compile --fqbn even if fqbn is not known,
which leads to more confusing errors. Abort earlier and explain what
to do (call the server with -fqbn instead).

Author: mbacarella

ls/builder.go

@@ -186,6 +186,12 @@ func (ls *INOLanguageServer) generateBuildEnvironment(ctx context.Context, fullB
 	ls.readLock(logger, false)
 	sketchRoot := ls.sketchRoot
 	config := ls.config
+
+	// Validate FQBN before attempting compilation
+	if config.Fqbn == "" {
+		ls.readUnlock(logger)
+		log.Fatal("FQBN (Fully Qualified Board Name) is required but not specified. Please restart the language server with the -fqbn flag or set default_fqbn in sketch.yaml")
+	}
 	type overridesFile struct {
 		Overrides map[string]string `json:"overrides"`
 	}