Was not running on my stacked PRs.

Small refactor for rest of stack.

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* #524
* #523
* __->__ #522

Small refactor.

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* #524
* __->__ #523

Easy linking back to the repo, plus some social proof (stars/forks etc).

litellm is a library that abstracts away details/differences for a lot
of model providers. Adding an extension, so that any provider can easily
be integrated.

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* #532
* __->__ #524

This pull request introduces the following changes:
1. **Exclude translated pages from search**: I explored ways to make the
search plugin work with the i18n plugin, but it would require extensive
custom JavaScript hacks. So for now, I’m holding off on this work.
2. **Switch from GPT-4.1 to o3 for even better translation quality**:
While 4.1 performs well, o3 shows even greater quality for this task,
and there’s no reason to avoid using it.

See #528, some folks are having issues because their output types are
not strict-compatible.

My approach was:
1. Create `AgentOutputSchemaBase`, which represents the base methods for
an output type - the json schema + validation
2. Make the existing `AgentOutputSchema` subclass
`AgentOutputSchemaBase`
3. Allow users to pass a `AgentOutputSchemaBase` to
`Agent(output_type=...)`

Closes #435 and closes #538.

Unit tests.

Only the file name is needed since graphviz's `render()` automatically
adds the file extension.
Also, unnecessary .gv (.dot) files are output, so the `cleanup=True`
option has been modified to prevent them from being saved.

Here is a similar modification, but in a different content.
- #451

## Summary
This replaces the default model provider with a `MultiProvider`, which
has the logic:
- if the model name starts with `openai/` or doesn't contain "/", use
OpenAI
- if the model name starts with `litellm/`, use LiteLLM to use the
appropriate model provider.

It's also extensible, so users can create their own mappings. I also
imagine that if we natively supported Anthropic/Gemini etc, we can add
it to MultiProvider to make it work.

The goal is that it should be really easy to use any model provider.
Today if you pass `model="gpt-4.1"`, it works great. But
`model="claude-sonnet-3.7"` doesn't. If we can make it that easy, it's a
win for devx.

I'm not entirely sure if this is a good idea - is it too magical? Is the
API too reliant on litellm? Comments welcome.

## Test plan

For now, the example. Will add unit tests if we agree its worth mergin.

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Co-authored-by: Steven Heidel <steven@heidel.ca>

…ech_group enum (#562)