fix: two indisputable test errors — non-URI delete id (051_02) + scalar type indexed as array (notifications)

Two small, indisputable test-side errors — both verified still present in develop, broker behaviour is correct in each case:

• 051_02_01 asserts 404 ResourceNotFound but DELETEs a bare 16-digit numeric id. A non-URI context id is 400 BadRequestData per TS 104-175 § 13.5.4, so a conformant broker never reaches the 404 path. Now uses urn:ngsi-ld:Context:<random> — a valid-but-unknown URI (the same shape as a real implicit-context id).
• NotificationUtils.Check Notification Data Entities collected [entities][0][type][0] — the first character of the scalar type string ("Building" → "B"), so the assertion failed with e.g. Building != B. EntityInfo.type is a single attribute name (TS 104-175 § 5.2.6.6.2; compacted to a string per TS 104-176 § 6.2.4). Dropped the trailing [0]. Affects the 047_* CSR-subscription notification assertions.