add completions for psql command

acceptance/cmd/psql/completions/out.test.toml

@@ -0,0 +1,5 @@
+Local = true
+Cloud = false
+
+[EnvMatrix]
+  DATABRICKS_CLI_DEPLOYMENT = ["terraform", "direct-exp"]

acceptance/cmd/psql/completions/output.txt

@@ -0,0 +1,6 @@
+
+=== Command should show instances names in autocomplete:
+my-database
+another-database
+:4
+Completion ended with directive: ShellCompDirectiveNoFileComp

acceptance/cmd/psql/completions/script

@@ -0,0 +1,2 @@
+title "Command should show instances names in autocomplete:\n"
+$CLI __complete psql ""

acceptance/cmd/psql/completions/test.toml

@@ -0,0 +1,10 @@
+[[Server]]
+Pattern = "GET /api/2.0/database/instances"
+Response.Body = '''
+{
+    "database_instances": [
+        {"name": "my-database"},
+        {"name": "another-database"}
+    ]
+}
+'''

cmd/psql/psql.go

@@ -32,13 +32,7 @@ You can pass additional arguments to psql after a double-dash (--):
 `,
 	}
 
-	// Wrapper for [root.MustWorkspaceClient] that disables loading authentication configuration from a bundle.
-	mustWorkspaceClient := func(cmd *cobra.Command, args []string) error {
-		cmd.SetContext(root.SkipLoadBundle(cmd.Context()))
-		return root.MustWorkspaceClient(cmd, args)
-	}
-
-	cmd.PreRunE = mustWorkspaceClient
+	cmd.PreRunE = root.MustWorkspaceClient
 	cmd.RunE = func(cmd *cobra.Command, args []string) error {
 		ctx := cmd.Context()
 		w := cmdctx.WorkspaceClient(ctx)
@@ -83,5 +77,26 @@ You can pass additional arguments to psql after a double-dash (--):
 		return lakebase.Connect(cmd.Context(), databaseInstanceName, extraArgs...)
 	}
 
+	cmd.ValidArgsFunction = func(cmd *cobra.Command, args []string, toComplete string) ([]string, cobra.ShellCompDirective) {
+		err := root.MustWorkspaceClient(cmd, args)
+		if err != nil {
+			return nil, cobra.ShellCompDirectiveError
+		}
+
+		ctx := cmd.Context()
+		w := cmdctx.WorkspaceClient(ctx)
+		instances, err := w.Database.ListDatabaseInstancesAll(ctx, database.ListDatabaseInstancesRequest{})
+		if err != nil {
+			return nil, cobra.ShellCompDirectiveError
+		}
+
+		var names []string
+		for _, instance := range instances {
+			names = append(names, instance.Name)
+		}
+
+		return names, cobra.ShellCompDirectiveNoFileComp
+	}
+
 	return cmd
 }