fix(afspec): coerce Subtask.details string to list before validation (fixes #658)

[mickume]

Summary

Added a Pydantic field_validator with mode="before" on Subtask.details that coerces a bare string into a single-element list. This fixes spec generate consistently failing on the tasks artifact because the LLM produces details as a string instead of list[str], causing a PydanticValidationError before the repair flow can run.

Closes #658

Changes

File	Change
packages/afspec/afspec/models.py	Added field_validator("details", mode="before") on Subtask to coerce str → [str]
packages/afspec/tests/test_models.py	Added 6 tests covering string coercion, list passthrough, empty list, default, model_validate, and nested TaskGroup deserialization

Tests

• TestSubtaskDetailsCoercion::test_string_coerced_to_list — bare string becomes single-element list
• TestSubtaskDetailsCoercion::test_list_unchanged — existing list input passes through
• TestSubtaskDetailsCoercion::test_empty_list_unchanged — empty list preserved
• TestSubtaskDetailsCoercion::test_default_is_empty_list — default factory unaffected
• TestSubtaskDetailsCoercion::test_model_validate_with_string_details — model_validate coerces string
• TestSubtaskDetailsCoercion::test_model_validate_nested_in_task_group — works through nested TaskGroup

Verification

• All existing tests pass: ✅ (5731 passing, same 14 pre-existing failures)
• New tests pass: ✅ (6/6)
• Linter / formatter: ✅
• No regressions: ✅

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