fix(segments): warn before discarding an unsaved feature-specific seg…

[narendraio]

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Changes

Closes #5368

When creating a feature-specific segment from the feature panel, the segment is
built in an inline drawer that is closed via the onCancel prop. That path
bypasses the modal's intercept-close handler, so closing the drawer silently
discarded any unsaved work.

This routes the drawer's cancel action through the existing onClosing guard,
so the user is asked to confirm before their unsaved changes are discarded —
the same confirmation already used when closing the full segment modal.

How did you test this code?

Manually:

1. Open a feature, go to the Segment Overrides tab and click to create a new
   feature-specific segment.
2. Enter a name / add a rule, then click Cancel on the drawer.
3. Confirm a "Discard changes" prompt now appears; choosing Cancel keeps the
   drawer open, choosing Ok discards and closes it.
4. Confirm that closing the drawer with no changes still closes immediately
   (no prompt).

Also ran npm run typecheck and eslint on the changed file (no new errors).

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