Python: Empty tool results lose correlation and fail serialization

Describe the bug

ChatHistory.add_tool_message() drops an empty-string tool result instead of preserving it as FunctionResultContent. The appended message still has role tool, but its items list is empty and the supplied tool_call_id is lost. Serializing that otherwise ordinary history message then raises IndexError because ChatMessageContent.to_dict() expects a tool message to contain a function result.

An empty string is a valid tool result for cases such as a successful command with no stdout or a no-op tool. The helper documents content as a string and requires tool_call_id; it should retain both even when the string is empty.

To reproduce

from semantic_kernel.contents.chat_history import ChatHistory

history = ChatHistory()
history.add_tool_message("", tool_call_id="call_123")

message = history.messages[-1]
print(message.role.value, len(message.items))
print(message.to_dict())

On current main (3438d882):

tool 0
IndexError: list index out of range

No model, provider, credentials, or network request is involved.

Root cause and proposed scope

ChatHistory._prepare_for_add() currently constructs FunctionResultContent only when content is truthy:

if role == AuthorRole.TOOL and content and not items:

Changing that predicate to content is not None preserves the existing behavior while allowing a valid empty result to retain result="", id, and call_id. A focused regression should assert both the FunctionResultContent fields and the serialized tool-message dictionary.

This is related to, but distinct from, #13678: that PR improves an Azure adapter's error for manually constructed tool messages with no items. This issue is about preventing the public ChatHistory.add_tool_message() helper from creating that invalid shape when its documented string input is empty.

Expected behavior

The helper should append one FunctionResultContent with the empty result and supplied call ID, and message.to_dict() should produce a valid tool message instead of raising.

Platform

• Language: Python
• Source: current repository main at 3438d882
• AI model: not applicable
• OS: macOS; provider-free reproduction

I can submit the focused predicate change and regression once maintainers confirm this contract.

Start in semantic_kernel/contents/chat_history.py at ChatHistory._prepare_for_add(), then reproduce the empty-result case from the issue. Add a focused regression covering the FunctionResultContent fields and message.to_dict(); done means an empty tool result retains its call ID and serializes without raising.