feat: drop legacy raw-X12 fallback; require pyx12 codes (PR 5 of 6)

[azoner]

Fifth slice of the X12 error code generalization plan. Now that every producer in the codebase emits pyx12 codes (#175-#178), the legacy raw-X12 fallback paths in the visitors and the validation tuples in error_item.py are dead. This PR removes them and tightens EleError/SegError __post_init__ to require ERROR_CODES keys.

Visitor changes

• pyx12/error_999.py visit_seg / visit_ele: drop the valid_IK3_codes / valid_IK4_codes tuples and the elif fallback. Visitors now only do ERROR_CODES.get(err_cde) — None spec or None ik_code = drop.
• pyx12/error_997.py visit_seg / visit_ele: same shape, drop valid_AK3_codes / valid_AK4_codes tuples.

Validation tightening

pyx12/error_item.py:

• EleError.__post_init__: was if err_cde not in ele_errors and err_cde not in ERROR_CODES: raise. Now if err_cde not in ERROR_CODES: raise. Error message updated to point to pyx12.error_codes.ERROR_CODES.
• SegError.__post_init__: same shape change.
• Removed the module-level seg_errors / ele_errors tuples (unused).
• Kept isa_errors and ISAError.__post_init__ — ISA-level codes are TA1 codes, not migrated to pyx12 codes (out of scope per the plan).

x12_code_for in error_codes.py keeps its permissive behavior of falling through to err_cde for unknown codes — needed by the JSON visitor for ISA/GS/ST envelope-level codes (\"025\", \"010\", etc.) which aren't migrated to pyx12 codes.

Test updates (test_error_item.py)

All SegError / EleError construction tests migrated to pyx12 codes:

• \"1\" → \"SEG_1_segment_not_found\", \"8\" → \"SEG_8_trailing_terminators\", etc.
• \"1\" → \"ELE_1_mandatory_missing\", \"10\" → \"ELE_10_not_used\", etc.

Two new tests assert that legacy raw-X12 codes are now rejected: test_invalid_code_legacy_raw_X12_rejected on both SegError and EleError.

Test plan

• pytest pyx12/test/: 581 passed (579 baseline + 2 new "legacy rejected" tests)
• mypy --strict pyx12: clean (88 files)
• ruff check + format --check: clean

Behavior change

This PR is the breaking-change gate for the refactor. Any external consumer constructing SegError / EleError directly with raw X12 codes will now hit EngineError at construction time. Tag for 4.1.0.

The X12 ack output (997 / 999) and JSON output remain unchanged — the table-driven visitor lookup produces the same X12 codes as before.

Plan reference

~/.claude/plans/radiant-petting-steele.md — see PR 5 section.

🤖 Generated with Claude Code