feat(cli/invoke): add support for direct tool invocation from CLI (googleapis#2353)

## Description

This PR introduces a new subcommand, invoke, to the toolbox CLI. This
feature allows developers to execute tools defined in their
configuration directly from the command line.

- New Subcommand: Implemented invoke as subcommand, which handles tool
lookup, parameter unmarshaling from JSON, and invocation.
- Persistent Configuration Flags: Updated cmd/root.go to make flags like
--tools-file, --tools-folder, and --prebuilt persistent, allowing them
to be used with subcommands.
- Testing: Added unit tests for various scenarios
- Documentation: Created a new "how-to" guide for CLI tool testing and
updated the CLI reference documentation.

Author: helloeve

cmd/invoke_tool_test.go

@@ -0,0 +1,131 @@
+// Copyright 2026 Google LLC
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package cmd
+
+import (
+	"context"
+	"os"
+	"path/filepath"
+	"strings"
+	"testing"
+)
+
+func TestInvokeTool(t *testing.T) {
+	// Create a temporary tools file
+	tmpDir := t.TempDir()
+
+	toolsFileContent := `
+sources:
+  my-sqlite:
+    kind: sqlite
+    database: test.db
+tools:
+  hello-sqlite:
+    kind: sqlite-sql
+    source: my-sqlite
+    description: "hello tool"
+    statement: "SELECT 'hello' as greeting"
+  echo-tool:
+    kind: sqlite-sql
+    source: my-sqlite
+    description: "echo tool"
+    statement: "SELECT ? as msg"
+    parameters:
+      - name: message
+        type: string
+        description: message to echo
+`
+
+	toolsFilePath := filepath.Join(tmpDir, "tools.yaml")
+	if err := os.WriteFile(toolsFilePath, []byte(toolsFileContent), 0644); err != nil {
+		t.Fatalf("failed to write tools file: %v", err)
+	}
+
+	tcs := []struct {
+		desc    string
+		args    []string
+		want    string
+		wantErr bool
+		errStr  string
+	}{
+		{
+			desc: "success - basic tool call",
+			args: []string{"invoke", "hello-sqlite", "--tools-file", toolsFilePath},
+			want: `"greeting": "hello"`,
+		},
+		{
+			desc: "success - tool call with parameters",
+			args: []string{"invoke", "echo-tool", `{"message": "world"}`, "--tools-file", toolsFilePath},
+			want: `"msg": "world"`,
+		},
+		{
+			desc:    "error - tool not found",
+			args:    []string{"invoke", "non-existent", "--tools-file", toolsFilePath},
+			wantErr: true,
+			errStr:  `tool "non-existent" not found`,
+		},
+		{
+			desc:    "error - invalid JSON params",
+			args:    []string{"invoke", "echo-tool", `invalid-json`, "--tools-file", toolsFilePath},
+			wantErr: true,
+			errStr:  `params must be a valid JSON string`,
+		},
+	}
+
+	for _, tc := range tcs {
+		t.Run(tc.desc, func(t *testing.T) {
+			_, got, err := invokeCommandWithContext(context.Background(), tc.args)
+			if (err != nil) != tc.wantErr {
+				t.Fatalf("got error %v, wantErr %v", err, tc.wantErr)
+			}
+			if tc.wantErr && !strings.Contains(err.Error(), tc.errStr) {
+				t.Fatalf("got error %v, want error containing %q", err, tc.errStr)
+			}
+			if !tc.wantErr && !strings.Contains(got, tc.want) {
+				t.Fatalf("got %q, want it to contain %q", got, tc.want)
+			}
+		})
+	}
+}
+
+func TestInvokeTool_AuthUnsupported(t *testing.T) {
+	tmpDir := t.TempDir()
+	toolsFileContent := `
+sources:
+  my-bq:
+    kind: bigquery
+    project: my-project
+    useClientOAuth: true
+tools:
+  bq-tool:
+    kind: bigquery-sql
+    source: my-bq
+    description: "bq tool"
+    statement: "SELECT 1"
+`
+	toolsFilePath := filepath.Join(tmpDir, "auth_tools.yaml")
+	if err := os.WriteFile(toolsFilePath, []byte(toolsFileContent), 0644); err != nil {
+		t.Fatalf("failed to write tools file: %v", err)
+	}
+
+	args := []string{"invoke", "bq-tool", "--tools-file", toolsFilePath}
+	_, _, err := invokeCommandWithContext(context.Background(), args)
+	if err == nil {
+		t.Fatal("expected error for tool requiring client auth, but got nil")
+	}
+	if !strings.Contains(err.Error(), "client authorization is not supported") {
+		t.Fatalf("unexpected error message: %v", err)
+	}
+}