Preserve third party category on partial update

When category was omitted from an MCP, GraphQL, or CLI update payload,
ThirdPartyService.Update overwrote the stored category with OTHER. Only
apply category when the request explicitly includes it, matching other
optional fields on the same update path.

Add an MCP e2e test that updates name without category and asserts the
existing category is unchanged.

Signed-off-by: Cursor Agent <cursoragent@cursor.com>

Co-authored-by: Bryan FRIMIN <bryan@frimin.fr>

pkg/probo/third_party_service.go

@@ -408,8 +408,6 @@ func (s ThirdPartyService) Update(
 
 			if req.Category != nil {
 				thirdParty.Category = *req.Category
-			} else {
-				thirdParty.Category = coredata.ThirdPartyCategoryOther
 			}
 
 			if req.SecurityPageURL != nil {