fix: use getContent() instead of get() for Apps Script file retrieval

projects().get() only returns project metadata (title, scriptId, dates),
not file contents. This caused get_script_project to always show an empty
files list and get_script_content to return "File not found" for
container-bound scripts.

The correct method is projects().getContent() which maps to
GET /v1/projects/{scriptId}/content and returns all files with
their source code.

Fixes #441

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

tests/gappsscript/test_apps_script_tools.py

@@ -62,12 +62,19 @@ async def test_list_script_projects():
 async def test_get_script_project():
     """Test retrieving complete project details"""
     mock_service = Mock()
-    mock_response = {
+
+    # projects().get() returns metadata only (no files)
+    mock_metadata_response = {
         "scriptId": "test123",
         "title": "Test Project",
         "creator": {"email": "creator@example.com"},
         "createTime": "2025-01-10T10:00:00Z",
         "updateTime": "2026-01-12T15:30:00Z",
+    }
+
+    # projects().getContent() returns files with source code
+    mock_content_response = {
+        "scriptId": "test123",
         "files": [
             {
                 "name": "Code",
@@ -77,7 +84,8 @@ async def test_get_script_project():
         ],
     }
 
-    mock_service.projects().get().execute.return_value = mock_response
+    mock_service.projects().get().execute.return_value = mock_metadata_response
+    mock_service.projects().getContent().execute.return_value = mock_content_response
 
     result = await _get_script_project_impl(
         service=mock_service, user_google_email="test@example.com", script_id="test123"