Spells/Warlock: Fix Summon Sayaad effect handler - #32050

Changes proposed:

• Fix the effect handler for Summon Sayaad (366222). Effect 0 is SPELL_EFFECT_SUMMON_PET, but the script registers SPELL_EFFECT_DUMMY, so its handler is not called.
• Prevent the default summon and cast the existing Succubus or Incubus variant selected by the script.
• Add a regression test for the registered script's effect hook and default-effect suppression.

Issues addressed:

Summon Sayaad does not run its scripted Succubus/Incubus selection. No linked issue.

Tests performed:

• Builds with Clang 18 and MySQL, with warnings treated as errors: worldserver, bnetserver and tests.
• All 36 CTests pass in Release with static scripts and Debug with dynamic scripts and no PCH. The regression passes 5 assertions and fails without the fix.
• Checked effect 0 against client build 12.1.0.69933 and verified the existing spell_warl_summon_sayaad binding. No SQL update is needed.

Known issues and TODO list:

• Test pet creation and dismissal in-game. The automated test covers effect dispatch and default-effect suppression; it does not check pet creation or the random distribution.

Please review the effect hook, default-effect suppression and test fixture.