Python: fix(bedrock): don't send toolChoice when no tools are configured (#5172)

* fix(bedrock): don't send toolChoice when no tools are configured

BedrockChatClient was sending toolConfig.toolChoice even when no tools
were configured (tools=None). AWS Bedrock requires toolConfig.tools to
be present whenever toolChoice is specified, causing a 400 validation
error.

Only set toolChoice when tool_config has a 'tools' key present.

Fixes #5165

Signed-off-by: bahtya <bahtyar153@qq.com>

* test: add tests for toolChoice without tools

- test_prepare_options_tool_choice_auto_without_tools_omits_tool_config
- test_prepare_options_tool_choice_required_without_tools_omits_tool_config

Verifies that toolConfig is omitted when tool_choice is set but no
tools are provided, preventing ParamValidationError from Bedrock.

* fix: address maintainer feedback — remove stray test file, raise ValueError for required without tools

1. Remove test_addition.py — stray duplicate of tests already in
   python/packages/bedrock/tests/test_bedrock_client.py, missing all
   necessary imports and would fail with NameError.

2. Change tool_choice='required' handling to raise ValueError when no
   tools are configured instead of silently falling through. Using
   'required' without tools is a logical contradiction — the model
   must invoke a tool but none exist — so surfacing this as a
   ValueError helps callers catch the misconfiguration early.

3. Update the corresponding test to expect ValueError instead of
   silently omitted toolConfig.

---------

Signed-off-by: bahtya <bahtyar153@qq.com>

python/packages/bedrock/agent_framework_bedrock/_chat_client.py

@@ -413,10 +413,14 @@ class BedrockChatClient(
                     # Omit toolConfig entirely so the model won't attempt tool calls.
                     tool_config = None
                 case "auto":
-                    tool_config = tool_config or {}
-                    tool_config["toolChoice"] = {"auto": {}}
+                    if tool_config and "tools" in tool_config:
+                        tool_config["toolChoice"] = {"auto": {}}
                 case "required":
-                    tool_config = tool_config or {}
+                    if not (tool_config and "tools" in tool_config):
+                        raise ValueError(
+                            "tool_choice='required' requires at least one tool to be configured, "
+                            "but no tools were provided."
+                        )
                     if required_name := tool_mode.get("required_function_name"):
                         tool_config["toolChoice"] = {"tool": {"name": required_name}}
                     else:

python/packages/bedrock/tests/test_bedrock_client.py

@@ -137,3 +137,36 @@ def test_prepare_options_tool_choice_required_includes_any() -> None:
 
     assert "toolConfig" in request
     assert request["toolConfig"]["toolChoice"] == {"any": {}}
+
+
+
+def test_prepare_options_tool_choice_auto_without_tools_omits_tool_config() -> None:
+    """When tool_choice='auto' but no tools are provided, toolConfig must be omitted.
+
+    Without tools, setting toolChoice would cause a ParamValidationError from Bedrock.
+    """
+    client = _make_client()
+    messages = [Message(role="user", contents=[Content.from_text(text="hello")])]
+
+    options: dict[str, Any] = {
+        "tool_choice": "auto",
+    }
+
+    request = client._prepare_options(messages, options)
+
+    assert "toolConfig" not in request, (
+        f"toolConfig should be omitted when no tools are provided, got: {request.get('toolConfig')}"
+    )
+
+
+def test_prepare_options_tool_choice_required_without_tools_raises() -> None:
+    """When tool_choice='required' but no tools are provided, a ValueError must be raised."""
+    client = _make_client()
+    messages = [Message(role="user", contents=[Content.from_text(text="hello")])]
+
+    options: dict[str, Any] = {
+        "tool_choice": "required",
+    }
+
+    with pytest.raises(ValueError, match="tool_choice='required' requires at least one tool"):
+        client._prepare_options(messages, options)