test(extension_types): add computed_field and validator tests for pydantic (ITL-142)

[kurodo3]

Summary

Closes ITL-142.

Pydantic v2 BaseModel support was already fully implemented and automatically
registered in the default context (v0.1.json) — this PR fills the last two test
gaps identified during the ITL-142 verification pass:

• test_computed_field_not_stored: pins that @computed_field properties are
  excluded from the Arrow struct schema (only model_fields is walked, not
  model_computed_fields). Prevents a future refactor from accidentally including
  computed properties in Arrow serialization.
• test_validator_runs_on_decode: confirms that Pydantic validators run during
  reconstruction from Arrow storage (via Model(**kwargs)), and documents that
  non-idempotent validators may produce asymmetric round-trips — a Pydantic
  property, not an Orcapod bug.

No production code changes — both behaviours were already correct.

Test plan

• uv run pytest tests/test_extension_types/test_pydantic_logical_type_factory.py -v → 33 passed
• uv run pytest tests/test_extension_types/ tests/test_hashing/test_pydantic_dataclass_hashing.py -v → all passed

🤖 Generated with Claude Code

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.

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