fix: show tool names in history and return the model answer

[anxkhn]

Fixes #438.

Agent.stream stored each MCP call as {'tool': name, ...}, but called_tools_history.jinja reads tool.name. The next decide prompt showed a blank tool name, so the agent could not see what it already called.

When no further tools were needed, the complete event also replaced the last model response with Task completed. Clients that read the final artifact got the placeholder instead of the <Answer> body.

This change stores the history record under name (and includes isError in the template), keeps the last model response as the complete content, and stops emitting Step N / raw history strings as artifact text. Tests render the real Jinja template and check the complete event.

Tested with:

uv run --with pytest --directory samples/python/agents/a2a-mcp-without-framework pytest tests/test_tool_history.py -q