Add on_use_handlers dict field for backward compatibility with cogames

[aaln]

Summary

• Adds on_use_handlers: dict[str, Handler] field to GridObjectConfig for backward compatibility with cogames
• The dict is automatically merged into the singular on_use_handler field via firstMatch() in the existing model validator
• Fixes cogames import crash: pydantic_core._pydantic_core.ValidationError: Extra inputs are not permitted for on_use_handlers

Context

The standalone mettagrid repo replaced on_use_handlers (dict) with on_use_handler (singular AnyHandler | None) using composite FirstMatch/AllOf handler types. However, cogames extensively uses the dict form (on_use_handlers={...}) across extractors, hubs, junctions, cognitive substrate evals, and overcogged game configs.

This adds the dict field back as a convenience that bridges to the new singular API — the model validator merges the dict entries into a FirstMatch composite, preserving the same runtime behavior.

Test plan

• Verified GridObjectConfig(on_use_handlers={"solve": Handler()}) correctly merges into on_use_handler
• Verified single-entry dict collapses to plain Handler (via firstMatch flattening)
• Verified empty dict leaves on_use_handler as None
• Verified both on_use_handler and on_use_handlers merge correctly
• Verified AgentConfig (inherits from GridObjectConfig) also works
• Verified exact cogames exploration.py pattern works without error

Made with Cursor

[aaln]

This PR adds backward compatibility for the on_use_handlers dict API that cogames relies on extensively.

The standalone mettagrid repo introduced composite handler types (FirstMatch/AllOf) and replaced on_use_handlers: dict[str, Handler] with on_use_handler: AnyHandler | None. However, cogames still uses the dict form in ~20+ places (extractors, hubs, junctions, cognitive substrate evals, overcogged, gear stations). This causes a pydantic ValidationError: Extra inputs are not permitted on any cogames import.

The fix is 8 lines: it adds on_use_handlers back as a convenience field and merges it into on_use_handler via firstMatch() in the existing model validator. Single-entry dicts collapse to a plain Handler, multi-entry dicts become a FirstMatch composite, and if both fields are provided they merge correctly. No behavior change for anyone already using the singular on_use_handler API.