fix: use strict undefined check for thinkingBudget to preserve 0 value

The falsy check !result.thinkingBudget treated thinkingBudget: 0 as missing,
allowing the generationConfig fallback to overwrite it. Use explicit
=== undefined check instead. Adds test for the edge case.

Co-authored-by: greptile[bot] <greptile[bot]@users.noreply.github.com>