Add support for constants in function call schema

So to get the LLM to respect this, I had to put reiterate that it's a constant multiple times by:

• Setting const
• Setting enum with a single value
• Adding a note to the description.

But it was consistently behaving after that!

I also adjusted the way required constraints are managed so it's computed at the point of render rather than adding the property, so it respects and changes. I've also made isRequired always FALSE if it's a constant.

Failing tests.

I believe that's all tests fixed - failure was a runner issue.

I've slightly tweaked it to not automatically modify the required property. That can still be done by things that modify the definition though.

Oh, and I also added the ability to apply restrictions via the tool explorer, in line with what can be configured on agents in AI Agents. That makes it a lot easier to test!