Fail closed when local output violates its output schema

[adaamko]

Problem

Pydantic validation errors can currently be printed while the invalid local result is still returned, and dict schemas are treated as valid without equivalent runtime validation.

Direction

• Compile both Pydantic and dict output schemas into enforceable validators.
• Validate every locally produced result before considering it handled.
• On failure, return a structured local error and use the configured fallback when available.
• Never return a partially validated local result.

Acceptance

• Invalid Pydantic output is not returned as a successful local result.
• Invalid dict-schema output is detected.
• A configured fallback receives schema-invalid local requests.
• Without a fallback, the caller receives an explicit error/abstention outcome.
• Tests cover classification, extraction/NER, and transformation outputs.

Non-goals

• Redesigning the complete public result API in this issue.