[api-v2] mplementar soporte básico para fases grupales con formación de grupos aleatoria, y heterogénea referida a una pregunta específica.

[borjaguzman]

Extend the phase transition logic to support group phases. In this version, groups should be formed randomly and heterogeneously based on a specific question defined in the activity design.

Tasks

• Detect if the current phase requires group work
• Extract grouping configuration from the phase design (group size, question ID, algorithm)
• Implement or reuse logic for heterogeneous random group formation
• Assign students into groups accordingly
• Persist the group structure in the database
• Return appropriate response
• Add integration test