[Chen Yufeiyang (feiiiiii5)]

Description

Fixes #3341.

Clearing (or leaving at the placeholder) the converter_target select for a converter like ArabiziConverter failed with ArabiziConverter.converter_target: '' not found in DefaultInstanceRegistry … — an error that neither matches the converter's actual contract nor tells the user what to do.

Two root causes:

• A phantom target in the derived contract. ConverterIdentifier promotes every constructor parameter named converter_target to a TARGET reference. Eight converters only inherit Converter.__init__'s converter_target=None — a seam the base validates and discards so subclasses can forward a target to super().__init__() — yet the derived contract advertised a target for them, including the LLM badge projected from it: ArabiziConverter, ArabicPresentationFormConverter, BrailleConverter, CharacterSpaceConverter, FlipConverter, JsonStringConverter, UrlConverter, ZeroWidthConverter.
• An empty selection resolved as a registry name. A cleared field sends converter_target: "", and resolve_constructor_args looked "" up as a registry name.

The fix, along the lines Richard Lundeen (@richlundeen) described in #3038 ("a separate check of the converter's constructor requirements and registry metadata, plus a clearer validation error"):

• derive_parameters gains seam_base_type: a non-required reference declared by that base is a construction seam, excluded from a subclass's contract unless the subclass redeclares it. Required base references and class-declared optional ones stay. Registry._seam_base_type() defaults to None, and only ConverterRegistry opts in — deliberately per-registry, since for example the Scorer base also declares an optional chat_target that is not a seam.
• resolve_constructor_args treats an empty-string reference value as an unsupplied selection: the constructor default applies (including apply_defaults-registered defaults, the FEAT: Default converter targets to the adversarial target #3038 feature) instead of an empty-name lookup.

Metadata and creation derive through the same path, so the advertised contract and the build contract cannot drift.

scenario	base 85d4f0c3	head 8f3b1a21
ArabiziConverter metadata	target param + LLM badge	no target param, is_llm_based=False
ArabiziConverter + converter_target=""	'' not found in DefaultInstanceRegistry	Unknown parameter 'converter_target' for 'ArabiziConverter'
PuzzledConverter + converter_target=""	'' not found …	builds; _converter_target is None
TranslationConverter + converter_target=""	'' not found …	converter_target is required for TranslationConverter …

Related: #3038 (merged) added default-target behavior for genuinely LLM-backed converters; this fixes the different seam path that made eight non-LLM converters advertise a target at all.

Tests and Documentation

15 new unit tests in tests/unit/registry/ cover the seam exclusion (optional base reference dropped; required and subclass-redeclared references kept), the metadata of the eight converters, the class-declared targets staying advertised, an invariant that no advertised target comes from the base seam, and the create paths above. Verified red at base where applicable: the seam tests fail without seam_base_type, and each cleared-target build fails on base with the registry-lookup error.

tests/unit/registry + tests/unit/converter: identical 53 pre-existing environment failures on base and head; 3414 → 3429 passing. tests/unit/backend: identical outcomes base/head (2736 passing). ruff 0.17.0 check/format clean. No documentation change applies; the dialog renders whatever the registry publishes, so the field and badge disappear structurally.

Environment notes for the runs above

• The 53 pre-existing failures are pinyin/seeded-determinism tests needing model/data deps absent from my environment; base and head failure sets were compared item by item.
• tests/unit/registry/test_attack_technique_registry.py does not collect locally (no pyrit distribution metadata) on base either; excluded from both runs.
• JupyText/docs-build were not run locally.