[FIX] product_customerinfo_picking: support variant-level customer info

[weinni2000]

Problem

The _compute_product_customer_code method on stock.move only looked up
customer info at the product template level (product_tmpl_id.customer_ids).
It ignored product.customerinfo records linked to a specific product variant
(variant_customer_ids), so the customer reference/name was never filled for
variant-specific entries.

Additionally, the guard condition checked product_tmpl_id.customer_ids while
the @api.depends tracked product_id.customer_ids, causing a mismatch.

Fix

• Guard condition: check product_id.customer_ids (consistent with @api.depends)
• Template-level lookup: add product_tmpl_id filter to avoid cross-template matches
• Variant-level lookup: after the template-level result, look up
  variant_customer_ids filtered by partner and product variant; if found,
  override the template-level values so variant-specific info takes precedence

Test scenario

1. Create a product with variants (e.g. colour A, colour B)
2. Add a customer reference on the template level and a different one on a
   specific variant
3. Create a delivery order for that variant
4. Verify the stock move shows the variant-level customer reference,
   not the template-level one

🤖 Generated with Claude Code

[weinni2000]

@it-fact4lifetimesupport:
Can you review it?

[JordiMForgeFlow]

@weinni2000 I have opened PR #2413 where I actually use the search method from the base product_customerinfo module. This already considers the variants.