Add unit tests for ConversationManager

[philippe-ths]

Parent

Part of #34

Goal

Verify conversation session lifecycle, constraints, and integration triggers.

What to test

• Exchange limit enforcement (6-exchange cap)
• Adjacency validation (participants must be within 1 tile)
• NPC-NPC and Player-NPC dialogue flows
• Goal extraction trigger fires after conversation ends
• Session state transitions (start, exchange, end)

[philippe-ths]

Tracked but deferred — leaving open for future work.