test: drive 0016 fixtures 021-028 + add structured-output unit tests

Removes the deferred-fixture skip markers for the 8 structured-output
conformance fixtures (021–028). All pass against the OpenAIProvider
+ harness extensions landed in earlier commits.

Adds tests/unit/test_structured_output.py covering bits the
conformance fixtures don't exercise directly:

- validate_response_schema edge cases: non-dict, non-object top-level,
  missing type.
- strict_mode_supported: required-coverage rule, additionalProperties
  true, nested-object violation, anyOf branch violation, internal $ref
  resolution, unresolvable $ref, $ref cycle (self-referential schema).
- _derive_schema_name: title-when-present, hash-fallback, determinism,
  empty-title behavior.
- _augment_messages_with_schema_directive: prepend-when-no-system,
  extend-existing-system, caller-list-not-mutated, serialized-schema-
  substring.
- Pydantic-class overload: class-in returns validated BaseModel
  instance; pydantic ValidationError wraps in StructuredOutputInvalid;
  wire body produced from class equals wire body produced from the
  equivalent .model_json_schema() dict.
- uses_prompt_augmentation_fallback inspect property: False by
  default, True when constructor flag is set.

Author: chris-colinsky

tests/conformance/test_llm_provider.py

@@ -75,17 +75,6 @@
     "018-content-blocks-image-source-missing": "0015 multimodal images (PR-2)",
     "019-content-blocks-invalid-detail-value": "0015 multimodal images (PR-2)",
     "020-content-blocks-inline-image-missing-media-type": "0015 multimodal images (PR-2)",
-    # proposal 0016 — structured output (this PR; wired up later in the
-    # commit sequence). These rows are removed in the commit that drives
-    # the structured-output fixtures.
-    "021-structured-output-success": "0016 structured output (this PR; not yet wired)",
-    "022-structured-output-parse-failure": "0016 structured output (this PR; not yet wired)",
-    "023-structured-output-validation-failure": "0016 structured output (this PR; not yet wired)",
-    "024-structured-output-non-transient": "0016 structured output (this PR; not yet wired)",
-    "025-structured-output-with-tool-calls": "0016 structured output (this PR; not yet wired)",
-    "026-structured-output-openai-wire-mapping-native": "0016 structured output (this PR; not yet wired)",
-    "027-structured-output-openai-wire-mapping-fallback": "0016 structured output (this PR; not yet wired)",
-    "028-structured-output-no-schema-regression": "0016 structured output (this PR; not yet wired)",
 }