fix: add adaptive risk agents example with relative-import tests

[pragam-m25]

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• 🐛 Bug fix
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Summary

This PR adds a new diagnostic example demonstrating adaptive risk-taking agents.

Each agent:

• chooses between safe and risky actions,
• maintains a short memory of past outcomes,
• adapts its risk preference based on recent experience.

The example is intentionally minimal and uses only core Mesa primitives.

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Motivation

Many adaptive agent models in Mesa end up concentrating:

• decision-making,
• action execution,
• memory updates,
• and learning logic

inside a single agent.step() method.

This example is designed to make that complexity explicit, rather than hide it,
and to serve as a concrete, example-first input for discussions around
richer behavioral abstractions in Mesa.

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Scope

• New example only (no API or behavior changes)
• Uses current Mesa patterns (no deprecated schedulers, no DataCollector)
• Relies on model.random for reproducibility
• Includes minimal smoke tests to ensure the model initializes and steps

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Testing

• Added smoke tests verifying:
  • model initialization
  • stepping without errors
• Tests intentionally do not assert outcomes or dynamics

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Notes

This example is exploratory and educational by design.
It does not propose new abstractions or patterns,
but highlights current modeling constraints through a working example.

[pragam-m25]

@quaquel Thanks for the guidance earlier — moved this example to mesa-examples as suggested.