Fixes #23097: Prevent duplicate Cable Paths when Cable Terminations are unchanged (#23100)

* fix(dcim): Prevent path rebuild when Cable Terminations unchanged

Compare Cable Terminations against stored values instead of the empty
cache when checking for modifications, so a freshly loaded Cable that is
resaved with the same terminations no longer rebuilds its paths. Raise
the flag whenever update_terminations() force-recreates an end, since
the edit form warms the cache that gated it and a profile change then
tore every path down without rebuilding it. Add regression tests for
both.

Fixes #23097

* fix(dcim): Preserve cable end order when terminations unchanged

Compare cable terminations against stored values instead of potentially
stale prefetched relations when checking for modifications. Skip setting
terminations in the form's clean() when a saved cable's members are
unchanged, preserving the connector order assigned by the profile.