Add support for usage limits

[dmontagu]

Add UsageLimits and raise errors when they are exceeded.

This could use more tests before merging, I wouldn't say I'm confident it is working properly in all cases yet. But I'm also not confident enough in the approach to really want to put a lot more effort into testing yet. (Though I'll note I did already add one simple test, and updated the FunctionModel and TestModel to track token counts in the usage.)

Also needs docs.

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[samuelcolvin]

Otherwise I think this looks great.

[dmontagu]

Not done reviewing, but here's a few copy edits on the docs..

[dmontagu]

Once this is merged let's create an issue talking about related improvements we might make:

• Automatically setting max_tokens in requests based on the limits and current usage
• Automatically removing function tools from the list of choices provided to the model if you are on the last allowed request

I don't think we want to do either of those things unconditionally, but it's probably worth creating a tracking issue for discussion about whether/how to make such changes

[hyperlint-ai]

The style guide flagged several spelling errors that seemed like false positives. We skipped posting inline suggestions for the following words:

• [Ee]rroring