Fix PoisonedBehavior damage over time to respect poison resistance

[bobtista]

• Fixes [GEN][ZH] PoisonedBehavior damage over time ignores POISON resistance #1616

The original code used DAMAGE_UNRESISTABLE to prevent an infinite loop where the DoT damage would retrigger onDamage(), which would restart the poison effect. The bug was that chem suits didn't work.

• Changed DoT damage type from DAMAGE_UNRESISTABLE to DAMAGE_POISON so resistance is properly applied
• Set m_sourceID to INVALID_ID for DoT damage
• Updated onDamage() to ignore damage with INVALID_ID source, preventing the infinite loop

Testing

• Units with POISON resistance (Chemical Suits) now properly resist poison DoT damage
• Poison DoT no longer causes infinite loops
• External poison attacks still trigger poison effects correctly

Note - I don't think the DoT damage gives XP with this fix.