[1998x-stack]

Phase 4 Tool Factory validation.

• Replace top-level-only manual checks with standard Draft 2020-12 validator, supporting nested objects/arrays, required/additionalProperties, numeric bounds, combinators, internal $defs and supported formats.
• Validate schema correctness when a ToolDef is registered; reject top-level non-object schemas and external $ref/$dynamicRef targets to avoid untrusted network schema retrieval.
• Reject non-object arguments even under legacy empty {} schemas.
• Revalidate after HITL approves modified arguments; no invalid payload reaches a handler.
• Add jsonschema>=4.23,<5 dependency, broad contract tests and adjust one historical permissive test.

This is structural input validation, not an OS sandbox or a substitute for an explicit filesystem path contract.