fix(schema): always meta-validate generated schemas

[morluto]

Problem

PR #577 skipped Draft 2020-12 meta-schema validation for selected Pydantic-generated schemas. Its replacement trust classifier inspected __pydantic_core_schema__ metadata to distinguish default generation from custom output.

That is not a stable ownership boundary. Pydantic supports schema replacement and mutation through model methods, field extras, WithJsonSchema, nested types, custom core schemas, custom generators, and JSON-schema hooks. Tracking those internal representations recreates part of Pydantic's schema-generation machinery and can miss version-specific customization paths.

Change

• remove the trusted validator and Pydantic core-schema trust classifier
• route every register_model() result through the existing register() boundary
• retain canonical-byte caching in _validated_schema, so each exact schema is meta-validated and compiled once per process
• retain regressions proving malformed model overrides, field extras, and WithJsonSchema output are rejected before descriptor persistence

This follows python-jsonschema's documented boundary: validate the schema itself, construct the validator, then reuse that validator for payloads.

Validation

• uv run --locked pytest -n 0 tests/component/schemas — 18 passed
• focused Ruff and mypy checks — passed
• exact diff review — no actionable findings

The runtime-construction performance concern remains valid, but should be addressed through initialization/publication/lazy-loading ownership rather than by weakening schema validation.

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