Add v2.6 model interface

[bejaeger]

• Doesn't make 2.6 the default choice just yet.
• To be merged once models are uploaded.

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[gemini-code-assist]

Code Review

This pull request adds support for the v2.6 model, including new interfaces, constants, and configurations. The changes are mostly straightforward additions to support the new version. I've identified a couple of areas for improvement:

• There's an opportunity to refactor duplicated code in classifier.py and regressor.py where default options for different model versions are created. Using a dictionary lookup would make this code more maintainable.
• In inference_config.py, the latest model configuration should point to the new v2.6 preprocessor configs for consistency and to prevent future bugs.

Overall, the changes are well-structured and the addition of tests for the new version is great.

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