[FIX] stock: enforce sn constraint when ml done

Prevent duplicate SN through python constraint (requires SN to be
attempted to be created) instead of assigning existing one (and
causing a duplication). Steps to reproduce:

- create a SN tracked product + activate "storage locations"
- assign a SN to tracked product
- create a planned receipt for the tracked product and assign same SN
  (i.e. same name).

Warning will pop up about repeat SN, but it is still possible to
validate the picking and create a duplicate SN.

Also add an extra `limit=1` to lot search for performance/safety.

Bug was discovered while working on related task: 1924758

closes odoo/odoo#61801

Signed-off-by: William Henrotin <Whenrow@users.noreply.github.com>

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