Guard Trader result dropdown tooltip against stale state

[mcagnion]

Description of the problem being solved:

The Trader result dropdown tooltip can be evaluated after the sorted row state or backing result entry has changed, which can leave the callback pointing at missing entries and trigger nil accesses.

Description of the solution:

Add early-return guards in the resultDropdown tooltip callback when:

• the sorted row is missing
• the requested dropdown entry is missing
• the backing result entry is no longer available

This keeps the nominal path unchanged and only hardens the callback against transient state desynchronization.

Steps taken to verify a working solution:

• Added targeted tests in TestTradeQuery_spec.lua covering:
  • a missing sortedResultTbl[row_idx] row
  • a missing backing result entry for the selected dropdown item
• Ran both targeted tooltip guard tests successfully.

Notes:

• I did not reference a current open issue because I did not find one that clearly matches this exact callback/state combination.
• Historical Trader issues in the same area include Crash upon changing search method after searching for best jewel #5542 and Trade Error #5544, with related fixes in PoB Trader: Fix issues caused by sorting mode change #5552, but this PR is a narrower defensive follow-up rather than a direct duplicate.

[Nightblade]

Hi, please remember to use TAB indentation. ( spec/System/TestTradeQuery_spec.lua is spaced. )