Add IDbContextFactory support to fix DbContext disposal issues in DI scenarios

[Copilot]

Problem

When using the EFCore adapter with Dependency Injection (e.g., in ASP.NET Core applications), users encounter ObjectDisposedException when the Enforcer attempts to perform policy operations:

public class IndexModel(IEnforcerProvider enforcerProvider) : PageModel
{
    private readonly IEnforcer enforcer = enforcerProvider.GetEnforcer()!;
    
    public void OnGet()
    {
        this.enforcer.LoadPolicy();  // ObjectDisposedException!
    }
}

Error: ObjectDisposedException: Cannot access a disposed context instance.

This occurs because:

1. The DbContext is registered as scoped in the DI container
2. The Enforcer (typically singleton or longer-lived) holds a reference to the adapter which holds the context
3. When the request ends, the DI container disposes the DbContext
4. Subsequent operations on the Enforcer fail because the context is disposed

Solution

This PR adds support for IDbContextFactory<TContext> (available in EF Core 5.0+) to the EFCoreAdapter. When using the factory pattern, the adapter creates a fresh DbContext for each operation and properly disposes it, allowing the Enforcer to safely outlive individual request scopes.

Changes

1. New Constructor Overloads

// For .NET 5.0+
public EFCoreAdapter(IDbContextFactory<TDbContext> contextFactory)

2. On-Demand Context Creation
Each operation (LoadPolicy, AddPolicy, RemovePolicy, etc.) now:

• Creates a new context from the factory
• Performs the operation
• Disposes the context automatically

3. Backward Compatibility
Existing code using direct DbContext injection continues to work unchanged.

4. Cross-Version Support
Conditional compilation handles differences between EF Core 5.0 (synchronous factory only) and 6.0+ (async factory support).

Usage

// In Program.cs or Startup.cs
public class MyCasbinDbContext : CasbinDbContext<int>
{
    public MyCasbinDbContext(DbContextOptions<MyCasbinDbContext> options) 
        : base(options) { }
}

// Register the factory (recommended for DI)
builder.Services.AddDbContextFactory<MyCasbinDbContext>(options =>
    options.UseSqlServer(builder.Configuration.GetConnectionString("Default")));

// Configure Casbin with factory-based adapter
builder.Services.AddCasbinAuthorization(options =>
{
    options.DefaultModelPath = "model.conf";
    options.DefaultEnforcerFactory = (serviceProvider, model) =>
    {
        var factory = serviceProvider.GetRequiredService<IDbContextFactory<MyCasbinDbContext>>();
        var adapter = new EFCoreAdapter<int, EFCorePersistPolicy<int>, MyCasbinDbContext>(factory);
        return new Enforcer(model, adapter);
    };
});

Testing

Added comprehensive tests to verify:

• Basic CRUD operations with factory pattern
• Async operations and persistence
• Real-world DI scenario simulation where enforcer outlives contexts

All existing tests continue to pass, confirming backward compatibility.

Documentation

Updated README with:

• Complete DI usage guide
• ASP.NET Core integration examples
• Explanation of why IDbContextFactory is recommended
• Migration path from direct injection

Fixes #[issue_number]

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Original prompt

---

This section details on the original issue you should resolve

<issue_title>Having lots of issues with DI</issue_title>
<issue_description>Hello, I am trying to DI the enforcer and do things like Load/Add/Remove Policys.

    public class CasbinDbContext(DbContextOptions<CasbinDbContext> options)
        : CasbinDbContext<int>(options)
    {
    }

    builder.Services.AddDbContext<CasbinDbContext>(options =>
        options.UseSqlServer(builder.Configuration.GetConnectionString("Default")));

    // https://github.com/casbin-net/casbin-aspnetcore/blob/master/samples/WebApplicationWithEfcoreSample/Startup.cs
    builder.Services.AddCasbinAuthorization(options =>
    {
        options.DefaultModelPath = "model.conf";

        options.DefaultEnforcerFactory = (p, m) =>
            new Enforcer(m, new EFCoreAdapter<int>(p.GetRequiredService<CasbinDbContext>()));
        options.DefaultRequestTransformerType = typeof(BasicRequestTransformer);
    });

    // ...

    public class IndexModel(IEnforcerProvider enforcerProvider) : PageModel
    {
        private readonly IEnforcer enforcer = enforcerProvider.GetEnforcer()!;
        public void OnGet()
        {
            this.enforcer.LoadPolicy();
        }
    }

Error:

ObjectDisposedException: Cannot access a disposed context instance. A common cause of this error is disposing a context instance that was resolved from dependency injection and then later trying to use the same context instance elsewhere in your application. This may occur if you are calling 'Dispose' on the context instance, or wrapping it in a using statement. If you are using dependency injection, you should let the dependency injection container take care of disposing context instances. Object name: 'CasbinDbContext'.

</issue_description>

Comments on the Issue (you are @copilot in this section)

@hsluoyz @sagilio @sociometry @AsakusaRinne

Fixes #80

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[hsluoyz]

@sagilio please review

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