.Net: Fix a bunch of function calling issues

[stephentoub]

1. A recent change caused the execution settings passed into the chat completion service to be null, causing automatic function calling to stop working. This fixes that.
2. We were providing the original execution settings to child invocations. This isn't appropriate, as the settings were known to be relevant to the parent invocation but not necessarily to the child invocation. It's also possibly dangerous, since the settings are known to enable function calling, and that could lead to recursive function invocation ad nauseum. For now, better to just explicitly not pass down the execution settings.
3. We weren't validating that a function the model requested was actually one we told it about. We should be doing so; otherwise, it could end up hallucinating and invoking functions that do exist in the kernel but that we didn't ask it to use.
4. We weren't validating with EnableFunctions + auto-invoke that the supplied functions were actually in the kernel; if they're not, we're setting ourselves up for failure, as we won't be able to invoke such a function when the model requests it.
5. We weren't sending back error messages to the model. Every tool request needs a response.
6. We weren't protecting against runaway cases where the invocation of a prompt function triggered auto-invocation of a prompt function ad nauseum. With (2), it's now hard to get into such a situation, but if we do, this adds a backstop that limits the length of that chain and disables auto-invocation if it's hit.
7. We were logging the function requests after validating them rather than before, but from a logging perspective, it's better to get that information promptly.
8. The "required" function calling support (where you can force the model to request a particular function's invocation) isn't behaving as expected: it's sending back a tool request but with a "stop" finish reason and it then balks at attempts to send back a function response. Until we have a better understanding of why this is happening and the right recourse, I've marked the RequireFunction entrypoint to it as experimental.