Explicit parameter bindings for each tool.
The order and length of this list must match tool_config_ids. Each dict maps
parameter names to their ParameterBinding specifications.
If None, default bindings are computed based on tool parameter schemas:
- Has x-hidden-from-model with default → source="fixed", fixed_value=
- Has x-hidden-from-model without default → source="user"
- Otherwise → source="agent"