Fix `drop` (and other ops) on a never-written collection under lazy shards
Summary: Fix drop (and other ops) on a never-written collection under lazy shards (v0.6.0b3).
Lazy shard creation (0.6.0b2) makes a collection's documents shard exist only
once something is written to it, and the Rust server's drop_collection ran
purge_collection_tables unconditionally — which opened the collection's shard
cursor and failed with a WiredTiger "No such file or directory" error when the
collection had never been written (dropping an empty / never-created collection,
a no-op in MongoDB). The purge now treats an absent shard as "no rows to remove".
The gap escaped the test suite because tests always create a collection before
dropping it; it was caught by the standalone binary's PGO-profile workload, whose
first operation is coll.drop() on a fresh collection. A regression test now
exercises every operation (drop / find / count / distinct / delete / update /
aggregate / findAndModify / create-index-then-drop) against never-written
collections on the Rust server.