Add enforce_handoff configuration option to IDV session SDK

[Copilot]

Implements enforce_handoff configuration option for IDV Create Session to require mobile handoff completion.

Changes

• SdkConfig: Added EnforceHandoff nullable boolean property with JSON serialization as enforce_handoff
• SdkConfigBuilder: Added WithEnforceHandoff(bool) fluent method for builder pattern
• Validation: Constructor throws ArgumentException if enforce_handoff is true when allow_handoff is false
• Tests: Added unit tests covering validation logic and valid configuration scenarios

Usage

// Enforce mobile handoff (must also allow it)
var config = new SdkConfigBuilder()
    .WithAllowHandoff(true)
    .WithEnforceHandoff(true)  // Requires mobile completion
    .Build();

// This will throw ArgumentException
var invalid = new SdkConfigBuilder()
    .WithAllowHandoff(false)
    .WithEnforceHandoff(true)  // Invalid: can't enforce when not allowed
    .Build();

The validation ensures configuration consistency at SDK level before API submission. Default value is null for backward compatibility.

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