test: consolidate PROPERTY_MAPPING suites around a public behavior matrix (#799)

[TKaltofen]

Summary

Phase 7 cleanup for the PROPERTY_MAPPING hardening epic (os-006, was #799). Consolidates the PROPERTY_MAPPING test suite around an explicit public-behavior matrix and retires duplicate private-helper coverage, with no production change and no loss of unique regression protection.

Changes

• Behavior matrix doc (PROPERTY_MAPPING_TEST_MATRIX.md): maps each behavior axis (match path, option source, presence, required/conditional, value shape, value space, callback outcome, diagnostics, defaults lifecycle, construction) to its owning public-boundary test. Records the private-helper tests deliberately kept as intentional contracts, the retired seams, and the deferred fixture-parametrization work with its rationale.
• Centralize transitional seam-absence pins: move the _get_validation_function gone and _validate_type_validators -> _validate_match_guards rename pins out of test_property_mapping_unified_model.py into test_retire_transitional_seams.py, next to the os-005 seam-absence pins they belong with. The public field-default assertion is preserved on the fields test.
• Drop a private-lambda pin: test_property_mapping_container_invariance.py no longer pins the plugin element_validator lambda directly; its three public sibling tests and core test_property_mapping_sequence_unpacking.py own that per-element contract.
• Drop two shipped-plugin facade duplicates: the "reason is None for a valid or absent value" pair in test_property_mapping_enforced_validators.py; that contract is core-owned in test_name_path_validates_option_values.py.
• Remove the stale-name constants left unreferenced by the above.

Coverage preservation

Net change over the 15 PROPERTY_MAPPING test files: 529 -> 514 collected node ids. Every removed or moved test traces to a surviving public or core boundary that asserts the same behavior; the two seam-absence pins are relocated, not deleted. No new module-level FeatureGroup subclasses, so no added registry-leak surface.

Validation

• tox green: 7147 passed, 170 skipped (skip count unchanged), ruff format + check, pip-licenses, mypy --strict, and bandit all pass.
• Two independent deep reviews (a fresh Claude Opus reviewer and codex): both confirm no coverage silently dropped and no production behavior change. Matrix wording nits raised by the Opus review were fixed; the dropped element-validator-shape pin was accepted as intended per the ticket (not an intentional contract).

Test refactor only.