Add settings to Model base class

[svilupp]

Fixes #2119

The implementation follows the suggestion by @DouweM

This PR:

• implements a settings field in the Model class
• adapts the settings resolution to go in this order: model settings < agent model settings < run-specific model settings
• adds 2 updates to the docs to mention how to use this feature with FallbackModel and the new settings hierarchy
• tests for the settings handling - it's in a separate file as it's a separate behavior, or should I merge it with other tests?

# Issues Encountered

• Found various "special" model classes inheriting from the core Model superclass (instrumentedmodel, testmodel, wrappermodel, fallbackmodel)

I dediced on the following behaviour -- any thoughts?

• Wrapper/Fallback Models: Should use the underlying model's settings, not have their own
• Instrumented Models: Keep existing behaviour/fields unchanged (it literary has a field called settings, so I had to change the field definition to be self.settings: ModelSettings | InstrumentationSettings | None = settings
• TestModel: Added settings field for API consistency

Next Steps

• Agree the correct behavior/design for the complicated model classes
• Perhaps I could refactor some wrapper code (tried to resolve the right settings in agent.run()) to be a separate utility with its own tests?

[hyperlint-ai]

PR Change Summary

Enhanced the Model base class by introducing a settings field and updating the documentation to reflect the new settings hierarchy.

• Implemented a settings field in the Model class.
• Established a settings resolution order: model settings, agent model settings, and run-specific model settings.
• Updated documentation to explain the new settings feature and hierarchy.
• Added tests for settings handling.

Modified Files

• docs/agents.md
• docs/models/index.md

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[DouweM]

@svilupp Thanks for working on this!