fix: improve error message when beautifulsoup4/lxml not installed for load_web_page

Merge google#4853

## Description

The built-in `load_web_page` tool requires `beautifulsoup4` and `lxml`, which are available via the `[extensions]` optional dependency group. When a user installs `google-adk` without the `[extensions]` extra and calls `load_web_page`, they get a raw `ModuleNotFoundError: No module named 'bs4'` with no guidance on how to resolve it.

This change wraps the deferred imports in a `try/except` to provide a clear, actionable error message:

```
ImportError: load_web_page requires the "beautifulsoup4" and "lxml" packages.
Install them with: pip install google-adk[extensions]
```

Fixes google#4852

Co-authored-by: Liang Wu <wuliang@google.com>
COPYBARA_INTEGRATE_REVIEW=google#4853 from brucearctor:fix/load-web-page-import-error 27aa20e
PiperOrigin-RevId: 933416379

Author: 2 people

pyproject.toml

@@ -193,6 +193,7 @@ optional-dependencies.test = [
   "a2a-sdk>=0.3,<0.4",
   "anthropic>=0.78",                                                 # For anthropic model tests; 0.78 introduced ThinkingConfigAdaptiveParam (required for Claude Opus 4.7).
   "anyio>=4.9,<5",
+  "beautifulsoup4>=3.2.2",
   "crewai[tools]; python_version>='3.11' and python_version<'3.12'", # For CrewaiTool tests; chromadb/pypika fail on 3.12+
   "e2b>=2,<3",
   "gepa>=0.1",
@@ -220,6 +221,7 @@ optional-dependencies.test = [
   "langgraph>=0.2.60,<0.4.8",
   "litellm>=1.83.7,<=1.83.14",
   "llama-index-readers-file>=0.4",
+  "lxml>=5.3",
   "mcp>=1.24,<2",
   "openai>=1.100.2",
   "opentelemetry-exporter-gcp-logging>=1.9.0a0,<=1.12.0a0",

src/google/adk/tools/load_web_page.py

@@ -286,7 +286,14 @@ def load_web_page(url: str) -> str:
   Returns:
       str: The text content of the url.
   """
-  from bs4 import BeautifulSoup
+  try:
+    from bs4 import BeautifulSoup
+    import lxml  # noqa: F401 -- verify lxml is available for the parser
+  except ImportError as e:
+    raise ImportError(
+        'load_web_page requires the "beautifulsoup4" and "lxml" packages. '
+        'Install them with: pip install google-adk[extensions]'
+    ) from e
 
   try:
     response = _fetch_response(url)