Bluetooth: Shell: Fix GATT UUID race condition with per-operation storage

The static global UUID variable in the GATT shell module was shared across
all concurrent discover and read-by-uuid operations, creating a potential
race condition where one operation could overwrite the UUID before another
completed. Move UUID storage into the per-operation gatt_op_context
structure to isolate UUID state for each operation. This enables safe
concurrent operations without data corruption.

Signed-off-by: Evgenii Kosenko <Evgenii.Kosenko@silabs.com>