Add validation for ContractConfig and DiagramConfig

[Michael628]

Parent PRD

#15

What to build

Add validate_config functions for ContractConfig and DiagramConfig that reject invalid empty collections, and register them via validate=validate_config in their register_task calls.

pyfm/tasks/contract/contraction.py:

• Add validate_config(config) that raises ValueError if config.diagrams is an empty dict

pyfm/tasks/contract/diagram.py:

• Add validate_config(config) that raises ValueError if config.mesons is an empty list

Tests:

• Test that building ContractConfig with empty diagrams raises a validation error
• Test that building DiagramConfig with empty mesons raises a validation error

Acceptance criteria

• ContractConfig validation rejects empty diagrams dict with clear error message
• DiagramConfig validation rejects empty mesons list with clear error message
• Validation functions registered via validate=validate_config in register_task
• Tests cover both valid and invalid cases
• Existing tests still pass

Files in scope

• pyfm/tasks/contract/contraction.py (modify)
• pyfm/tasks/contract/diagram.py (modify)
• test/tasks/contract/test_contraction.py (modify or create)
• test/tasks/contract/test_diagram.py (modify or create)

Implementation guardrails

• Delete means delete. If this issue says to remove something, remove it entirely. Do not preserve the removed functionality in a different form (e.g. extracting it into helper functions or a new module) unless explicitly instructed.
• Preserve semantics when substituting. When replacing one pattern with another, verify the replacement has the same observable behavior: copy vs. mutate, return value, side effects. If the old code created a copy, the new code must also create a copy.
• No drive-by improvements. Do not refactor, reformat, or "improve" code that is not directly related to the acceptance criteria. No import reordering, no style changes, no new abstractions.

Blocked by

• Blocked by Uniform builder slicing with docstrings and default preprocessor pop #16 (needs uniform DICT _preprocessor routing for ContractConfig builds)

User stories addressed

• User story 6 (reject empty diagram/meson collections)