Parent epic: #304

Problem

The tool source generator maps parameter types to JSON schema types, but silently falls back to "string" for any type it doesn't recognize. If someone adds a DateTime, enum, List<string>, or custom type as a tool parameter, the generator:

1. Emits "type": "string" in the schema — the LLM gets no useful format hint
2. Generates ParseArguments code that will likely fail at runtime with a cast/parse error

This compiles cleanly and tests pass until someone actually invokes the tool with that parameter.

Supported types today

• string → "string"
• int, long → "integer"
• float, double → "number"
• bool → "boolean"
• Nullable variants of the above

Everything else silently becomes "string".

Proposed fix

Add a diagnostic to NetclawToolGenerator that emits a warning when a TParams constructor parameter uses an unsupported type:

NCLW2002: Tool parameter type is not supported by schema generator

• Trigger: TParams record constructor parameter is not one of the supported primitive types
• Severity: Warning
• Fix: Use a supported type, or add support for the new type in the generator

This is a generator enhancement, not a separate analyzer — the diagnostic should be emitted during source generation since the generator already inspects parameter types.

Acceptance criteria

• Generator emits diagnostic for unsupported parameter types
• Diagnostic includes the parameter name, type, and tool name
• Existing tools compile without new warnings
• If we later add support for new types (e.g., enums), the diagnostic stops firing for those