test: enforce pyx12 code prefix matches err_handler tree node type

[azoner]

Add unit-test invariants that pyx12 code prefixes match where the error attaches in the err_handler tree:

• ELE_* and COMP_* codes must land in err_ele.errors (composites surface as element-level errors).
• SEG_* codes must land in err_seg.errors.

Also fix the one place that violated this invariant: the historical PR #161 hardcode in apply_segment_errors that stored a literal \"8\" in err_seg.errors when collapsing element-level validator errors (too-many-elements, syntax violations, mandatory composite missing) to a spec-correct IK3 code. Replace with new pyx12 code SEG_8_HAS_DATA_ELEMENT_ERRORS (ak/ik=\"8\"); ack output unchanged because the visitor still resolves to \"8\" via the table, and the original element-level pyx12 code is preserved in the err_str.

New tests

pyx12/test/test_error_codes.py (new file):

• TableConsistency.test_every_code_has_a_known_prefix
• TableConsistency.test_prefix_matches_spec_level — the main invariant: every code's name prefix matches its declared ErrorCodeSpec.level
• TableConsistency.test_key_matches_spec_code
• TableConsistency.test_ak_and_ik_codes_when_set_are_short_strings
• X12CodeForResolution.test_known_code_resolves_to_ik_code
• X12CodeForResolution.test_unknown_code_falls_through — envelope-level codes (\"025\") passthrough
• X12CodeForResolution.test_ak_preferred_when_ik_is_none

pyx12/test/test_x12n_document.py (new ErrorCodePrefixPlacement class):

• test_walker_seg_errors_have_seg_prefix — uses bad_2010AA_bug
• test_validator_ele_errors_have_ele_or_comp_prefix — uses 834_lui_id_5010_non_ascii
• test_no_raw_x12_code_leaks_into_seg_or_ele_errors — parameterized across 7 fixtures

Updates

• pyx12/error_codes.py — new SEG_8_HAS_DATA_ELEMENT_ERRORS constant + ERROR_CODES entry
• pyx12/map_if/_segment.py apply_segment_errors — emit SEG_8_HAS_DATA_ELEMENT_ERRORS instead of literal \"8\"
• pyx12/test/test_map_if.py — ApplySegmentErrorsRouting test updated to expect the new code

Test plan

• pytest pyx12/test/: 597 passed + 7 subtests (581 + 6 from PR feat: add --suppress CLI flag + suppress_error_codes filter (PR 6 of 6) #180 + 10 new from this PR)
• mypy --strict pyx12: clean (89 source files; new test_error_codes.py)
• ruff check + format --check: clean
• res997 / resAck / resJson fixtures unchanged — the apply_segment_errors collapse path is not exercised by any existing fixture (regenerator helper run, no diffs)

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