feat: support models requiring strict alternating user-assistant roles

[Ricardo-M-L]

Summary

• Adds support for model APIs (DeepSeek R1, Mistral) that reject requests with consecutive same-role messages, as described in Support model APIs that require strict alternating user-assistant roles #5965
• Implements automatic message merging to ensure strict user-assistant alternation before sending to the LLM
• Detects affected models via both a new requires_alternating_roles field in ModelInfo and a family-based check in ModelFamily.requires_alternating_roles()

Changes

autogen-core (models)

• ModelInfo: Added optional requires_alternating_roles: bool field
• ModelFamily: Added requires_alternating_roles() static method that returns True for R1, MISTRAL, MINISTRAL, CODESRAL, OPEN_CODESRAL_MAMBA, and PIXTRAL families

autogen-agentchat (agents & teams)

• ensure_alternating_roles() utility function in utils/_utils.py:
  • Preserves leading system messages
  • Merges consecutive same-role UserMessages into one
  • Merges consecutive same-role AssistantMessages (including thought fields)
  • Handles FunctionExecutionResultMessage as user-role, merging when adjacent to other user-role messages
  • Converts non-leading SystemMessages to UserMessages
• AssistantAgent._get_compatible_context(): Now applies alternating role enforcement when the model requires it (via explicit flag or family detection)
• SelectorGroupChat._select_speaker(): Now applies alternating role enforcement to selector messages before sending to the model

Tests (test_alternating_roles.py)

• 15+ test cases covering:
  • Basic alternation (empty, single, already-alternating)
  • Consecutive same-role merging (user, assistant, 3+ messages, thought merging)
  • FunctionExecutionResultMessage handling
  • Non-leading system message conversion
  • Complex scenarios (multi-agent, tool calls, DeepSeek R1, selector retry)
  • ModelFamily.requires_alternating_roles() for all families
  • ModelInfo field presence/absence
  • Edge cases (only system messages, empty content, large sequences)

Fixes #5965

Test plan

• Verify ensure_alternating_roles correctly merges consecutive same-role messages
• Verify AssistantAgent applies alternation for models with requires_alternating_roles=True
• Verify SelectorGroupChat applies alternation for affected model families
• Verify no behavioral change for models that don't require alternation (GPT-4o, Claude, etc.)
• Run existing test suite to ensure no regressions

🤖 Generated with Claude Code

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