fix(agents): surface unknown workflow tool calls

[alexanderkreidich]

Summary

• stop ChatWorkflow when an assistant requests a tool name that is not registered on the workflow
• raise a descriptive error that includes the unknown tool name, tool call id, and registered tool names
• add regression coverage for both the default raise path and ErrorPolicy.RETURN

Testing

• uv run pytest libs/giskard-agents/tests/test_workflow_error_handling.py -k unknown_tool_call -q
• uv run pytest libs/giskard-agents/tests/test_workflow_error_handling.py -q
• make check
• make test-unit PACKAGE=giskard-agents

Closes #2413

[gemini-code-assist]

Code Review

This pull request improves error handling in the ChatWorkflow by raising a ValueError when an unknown tool is called, replacing the previous behavior of silently skipping such calls. The error message now provides helpful context, including the name of the missing tool and a list of available tools. Additionally, new test cases were added to ensure that these errors are correctly handled and reported. I have no feedback to provide.