[DNM] Fix panic when sorting non-SQL lists by a boolean field (rancher/rancher#55593)

[rohitsakala]

Problem

Sorting a non-SQL list by a boolean field panics and kills the request handler, so Cluster Explorer renders blank:

http: panic serving ...: interface conversion: interface {} is bool, not string
    listprocessor.SortList(...) processor.go:383

Cause

SortList did an unchecked val.(string) on values from data.GetValueFromAny, but sortable fields aren't always strings — e.g. metadata.state.error (a bool, behind the dashboard's State column). Regressed in #618, which dropped the earlier convert.ToString(...). Non-SQL path only; the SQL cache sorts via ORDER BY.

Fix

Restore convert.ToString(val); add a TestSortList regression case for boolean/missing fields.

Fixes rancher/rancher#55593

[crobby]

Looks good