fix: stop Freaky Forester pheasants granting combat experience

[HarleyGilpin]

Problem

Killing pheasants during the Freaky Forester random event grants full combat experience (attack style xp + Constitution), which can disrupt goals for skiller builds. On rs wiki behavior the event pheasants give no combat experience.

Solution

• Adds a combat_exp npc definition param (default true), checked once in CombatExperience.grant so all xp paths (melee/magic/range, Constitution, familiars) respect it.
• Sets combat_exp = false on the pheasant base definition; all variants inherit it via clone.
• The flag is reusable for any other npc that shouldn't give combat xp.

Testing

• New WorldTest: kills a pheasant through real combat and asserts Attack/Strength/Constitution experience is unchanged.
• Negative-checked: with the flag flipped to true the test fails at the xp assertion, so the guard is what's being exercised.
• Existing FreakyForesterTest and CombatTest suites pass.