UI: Fix mapped task XCom navigation from Grid #64875

[vishakha1411]

Fixes #64875

What changed

This fixes UI navigation when switching between dynamically mapped tasks from Grid while viewing a task-instance tab such as XCom.

Previously, navigating from a mapped task instance page like:

/tasks/task_1/mapped/0/xcom

to another mapped task from Grid could preserve the xcom sub-route even when no concrete mapped index was selected for the destination task. That could produce an invalid route and result in map_index=NaN.

Result

Instead of failing with an invalid map_index, navigation now lands on:

/tasks/<task_id>/mapped i.e

which lets the user choose a mapped task instance before opening XCom or other task-instance tabs.

I have not added a regression test in this PR yet. If maintainers prefer, I can add one in a follow-up or update this PR.

[bbovenzi]

Looks good. Let's add a quick regression test in this PR.

[vishakha1411]

I have added a test case in links.test.ts for this navigation path to check that switching to another mapped task without a selected map index goes to the mapped task overview. This makes sure we don’t preserve the invalid xcom sub-route.