[TEA] Add MultiAgentEvolutionResult serialization parity tests

[Alberto-Codes]

TEA Action Item

Parent Audit: #282
Category: Coverage Distribution
Priority: P2 — Notable gap, should fix soon
Effort: S — Under 30 minutes

Problem

MultiAgentEvolutionResult has only 3 serialization tests vs 14 for EvolutionResult. Missing coverage includes:

• Invalid stop_reason handling
• Missing required field raises
• Default values for optional fields
• Empty dict handling
• JSON serializable verification

Impact

• Asymmetric confidence: EvolutionResult serialization is well-tested but MultiAgent variant has gaps
• Multi-agent is the more complex type (nested agent_results dict) — deserves at least parity coverage

Proposed Fix

1. Port the following test patterns from EvolutionResult to MultiAgentEvolutionResult:
   • Invalid stop_reason raises ConfigurationError
   • Missing required field raises
   • Empty dict raises
   • JSON serializable check
   • Default optional field values

Files Likely Touched

• tests/unit/domain/test_models.py

Acceptance Criteria

• MultiAgentEvolutionResult has at least 8 serialization tests (up from 3)
• All error-path patterns from EvolutionResult are mirrored
• All existing tests pass (uv run pytest)

Related Issues / Context

• Parent audit: [TEA Audit] Coverage Distribution #282
• Found during: TEA content review of Story 2.2 serialization tests