fix: keep ajax callbacks scoped to their originating modal

[dusan-maintains]

Summary

• bind AJAX success/fail handling to the originating modal instance instead of the current top modal
• avoid removing/hiding the wrong modal when asynchronous requests resolve out of order
• safely no-op when the originating modal is already closed

Why

Current callbacks use getCurrent() / modals.pop(), which can target the wrong modal in race conditions (stacked modals or modal closed before AJAX resolution).

Changes

• jquery.modal.js

Test Plan

• trigger two AJAX modals in close succession and verify each response updates only its own instance
• close the originating modal before AJAX completion and verify no unrelated modal is modified
• verify normal single-AJAX-modal behavior is unchanged

Compatibility

• no API changes
• behavior change only in race/edge paths to preserve correct instance ownership

[dusan-maintains]

Maintainer note: this change is scoped to async race/ownership paths only (no API/default changes). If useful, I can add a minimal reproducible snippet in the PR description.