Add comprehensive example for Optimize set_on_model callback functionality

[Copilot]

This PR adds a new comprehensive example demonstrating how to use the set_on_model callback feature with Z3's Optimize class. The example was requested in issue #207 to help users understand how to monitor optimization progress and implement custom termination conditions.

What's Added

A new example file 06 - Optimize with Model Callbacks.md under the Python Example Programs section that includes:

Basic Usage: Simple callback setup for monitoring optimization progress

def on_model(m):
    print("Found model:", m)
    sys.stdout.flush()
    
opt.set_on_model(on_model)

Multiple Objectives: Resource allocation example showing how to track model evolution with competing optimization goals

Early Termination: Custom monitoring class demonstrating how to implement termination logic based on model count or quality thresholds

MaxSAT Integration: Practical weighted MaxSAT example with solution collection and analysis, particularly useful with local search engines

External Files: Implementation based on the original issue's bare bones example, showing integration with SMT-LIB files and various optimization engine configurations

Key Benefits

• Provides working code examples for monitoring optimization progress
• Shows practical use cases for the callback functionality
• Demonstrates integration with different optimization engines (MaxSAT, local search)
• Includes the exact example from the issue description
• Follows existing documentation structure and formatting

The examples show how set_on_model callbacks are particularly valuable when using iterative optimization engines that find many intermediate solutions during the search process.

Fixes #207.

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