[19.0][FIX] purchase_order_line_description: preserve manual descriptions

[chienandalu]

The purchase line widget re-added the product name when users edited a line description, even though this module deliberately uses only the purchase description:

• Keep manually entered purchase descriptions unchanged and
• Added a regression tour.

https://www.loom.com/share/bd1acbb99f7943458eefcaa00df311f1

please review @yajo @fcvalgar

twin PR:

• [19.0][FIX] sale_order_line_description: description update sale-workflow#4437

cc @moduon MT-15180

[chienandalu]

please review @fcvalgar @yajo

[fcvalgar]

Great work @chienandalu! The fix behaves as expected and the manually entered purchase order line descriptions are now preserved correctly.

The following tests were performed:
Test 1: OK - A product with a purchase description was added to a request for quotation, and only the purchase description was displayed without the product name.
Test 2: OK - The purchase order line description was edited manually, and the entered text was preserved without re-adding the product name.
Test 3: OK - The request for quotation was saved and reloaded, and the manually edited description remained unchanged.
Test 4: OK - The description was modified after confirming the purchase order, and the new manual value was preserved correctly.
Test 5: OK - The purchase order line description was cleared manually, and the product name was not automatically restored.
Test 6: OK - Section and note lines continued to work correctly and were not affected by the change.
LGTM

[OCA-git-bot]

This PR has the approved label and has been created more than 5 days ago. It should therefore be ready to merge by a maintainer (or a PSC member if the concerned addon has no declared maintainer). 🤖