Enforce resolved tool argument constraints before every dispatch

• Status: Todo
• Kind: Bug
• Authors: jp
• Date: 2026-08-21
• Label: domain=llm
• Label: package=jp_llm
• Label: type=bug

Tool-call validation checks missing and unknown names, but does not enforce parameter types, enums, array item schemas, or nested value constraints. It runs only for local tools; MCP and built-in tools bypass it.

The recorded provider fixtures already contain invalid calls that pass through the test harness, including Cerebras arrays with numeric items, Google values outside the declared enum, and llama.cpp JSON-looking strings where an array was intended.

conversation.tools.<name>.parameters documents enums as allowed-value constraints and supports forcing a value. Those constraints must be enforced by JP rather than treated only as model guidance.

Open PR #998 validates resolved schema definitions but does not validate argument instances at dispatch time.

Acceptance criteria:

• Validate required fields, unknown fields, JSON types, complete-value enums, items, and nested properties against the resolved tool schema.
• Apply the same validation before local, MCP, and built-in dispatch.
• Normalize strict-provider null placeholders back to omission or the configured default before validation. Optional nullable values must not leak into a tool whose source schema does not accept null.
• Return the existing invalid-arguments tool response without invoking the target.
• Add tests using invalid calls taken from the Cerebras, Google, and llama.cpp fixtures.
• Add tests proving MCP and built-in parameter overrides are enforced.