Add accuracy test suite for solver methods

[jc-macdonald]

Extract the accuracy test suite from PR #18 into its own PR.

Scope

• tests/op_engine/test_core_solver_accuracy.py (~275 lines of additions)
• Address inline review feedback:
  • Make test helpers non-private (no leading underscore — these are test files, not public API)
  • Remove single-use constants from @pytest.mark.parametrize
  • Use direct import: from op_engine.core_solver import MethodName
  • Address PLR0914 (too-many-locals) properly instead of blanket suppression in pyproject.toml

Related

• Add new solver stepping methods #27 (solver methods must land first)
• Add accuracy test suite for solver methods #28 (promote reusable error-order helpers)

Ref: PR #18 review by @TimothyWillard