anthropic[patch]: add tests for secret_str for api key (#20986)

**Description:** Add tests to check API keys are masked
**Issue:** Resolves
https://github.com/langchain-ai/langchain/issues/12165 for Anthropic
models
**Dependencies:** None

libs/partners/anthropic/tests/unit_tests/test_chat_models.py

@@ -9,6 +9,7 @@ from langchain_core.messages import AIMessage, HumanMessage, SystemMessage, Tool
 from langchain_core.outputs import ChatGeneration, ChatResult
 from langchain_core.pydantic_v1 import BaseModel, Field, SecretStr
 from langchain_core.tools import BaseTool
+from pytest import CaptureFixture, MonkeyPatch
 
 from langchain_anthropic import ChatAnthropic
 from langchain_anthropic.chat_models import (
@@ -419,3 +420,53 @@ def test__format_messages_with_list_content_and_tool_calls() -> None:
     )
     actual = _format_messages(messages)
     assert expected == actual
+
+
+def test_anthropic_api_key_is_secret_string() -> None:
+    """Test that the API key is stored as a SecretStr."""
+    chat_model = ChatAnthropic(
+        model="claude-3-opus-20240229",
+        anthropic_api_key="secret-api-key",
+    )
+    assert isinstance(chat_model.anthropic_api_key, SecretStr)
+
+
+def test_anthropic_api_key_masked_when_passed_from_env(
+    monkeypatch: MonkeyPatch, capsys: CaptureFixture
+) -> None:
+    """Test that the API key is masked when passed from an environment variable."""
+    monkeypatch.setenv("ANTHROPIC_API_KEY ", "secret-api-key")
+    chat_model = ChatAnthropic(
+        model="claude-3-opus-20240229",
+        anthropic_api_key="secret-api-key",
+    )
+    print(chat_model.anthropic_api_key, end="")  # noqa: T201
+    captured = capsys.readouterr()
+
+    assert captured.out == "**********"
+
+
+def test_anthropic_api_key_masked_when_passed_via_constructor(
+    capsys: CaptureFixture,
+) -> None:
+    """Test that the API key is masked when passed via the constructor."""
+    chat_model = ChatAnthropic(
+        model="claude-3-opus-20240229",
+        anthropic_api_key="secret-api-key",
+    )
+    print(chat_model.anthropic_api_key, end="")  # noqa: T201
+    captured = capsys.readouterr()
+
+    assert captured.out == "**********"
+
+
+def test_anthropic_uses_actual_secret_value_from_secretstr() -> None:
+    """Test that the actual secret value is correctly retrieved."""
+    chat_model = ChatAnthropic(
+        model="claude-3-opus-20240229",
+        anthropic_api_key="secret-api-key",
+    )
+    assert (
+        cast(SecretStr, chat_model.anthropic_api_key).get_secret_value()
+        == "secret-api-key"
+    )