community[minor]: Improve Brave Search Tool, allow api key in env var (#30364)

- **Description:** 

- Make Brave Search Tool consistent with other tools and allow reading
its api key from `BRAVE_SEARCH_API_KEY` instead of having to pass the
api key manually (no breaking changes)
- Improve Brave Search Tool by storing api key in `SecretStr` instead of
plain `str`.
    - Add unit test for `BraveSearchWrapper`
    - Reflect the changes in the documentation
  - **Issue:** N/A
  - **Dependencies:** N/A
  - **Twitter handle:** ivan_brko

libs/community/tests/unit_tests/utilities/test_brave_search.py

@@ -0,0 +1,32 @@
+from typing import Any
+
+import pytest
+from pydantic import SecretStr
+
+from langchain_community.utilities.brave_search import BraveSearchWrapper
+
+
+def test_api_key_explicit() -> None:
+    """Test that the API key is correctly set when provided explicitly."""
+    explicit_key = "explicit-api-key"
+    wrapper = BraveSearchWrapper(api_key=SecretStr(explicit_key), search_kwargs={})
+    assert wrapper.api_key.get_secret_value() == explicit_key
+
+
+def test_api_key_from_env(monkeypatch: Any) -> None:
+    """Test that the API key is correctly obtained from the environment variable."""
+    env_key = "env-api-key"
+    monkeypatch.setenv("BRAVE_SEARCH_API_KEY", env_key)
+    # Do not pass the api_key explicitly
+    wrapper = BraveSearchWrapper()  # type: ignore[call-arg]
+    assert wrapper.api_key.get_secret_value() == env_key
+
+
+def test_api_key_missing(monkeypatch: Any) -> None:
+    """Test that instantiation fails when no API key is provided
+    either explicitly or via environment."""
+    # Ensure that the environment variable is not set
+    monkeypatch.delenv("BRAVE_SEARCH_API_KEY", raising=False)
+    with pytest.raises(ValueError):
+        # This should raise an error because no api_key is available.
+        BraveSearchWrapper()  # type: ignore[call-arg]