RPOPLPUSH is unimplemented (unknown command) while LMOVE and BRPOPLPUSH work

[TinDang97]

RPOPLPUSH is not implemented at all — no dispatch arm, no COMMAND INFO entry — even though LMOVE and BRPOPLPUSH both work.

127.0.0.1:7805> RPUSH src a b c
(integer) 3
127.0.0.1:7805> RPOPLPUSH src dst
(error) ERR unknown command 'RPOPLPUSH', with args beginning with:
127.0.0.1:7805> LMOVE src dst RIGHT LEFT
"c"
127.0.0.1:7805> BRPOPLPUSH src dst2 0.1
"b"
127.0.0.1:7805> COMMAND INFO rpoplpush
(empty array)

Real Redis 8.6.1 answers RPOPLPUSH src dst with the popped element.

Why this matters

RPOPLPUSH is deprecated in favour of LMOVE but is not removed, and it is
the form baked into a decade of client code and every reliable-queue tutorial.
redis-py, jedis, go-redis and node-redis all expose it as a first-class
method, so a client calling r.rpoplpush(...) gets an unknown-command error
rather than a working queue.

Where it is half-present

The name is already in the routing/metrics tables, which is what makes the gap
easy to miss:

• src/workspace/mod.rs:184 — listed in the command table
• src/workspace/mod.rs:281 — comment lists it as a two-key command
• src/admin/metrics_setup.rs:540 — has a metrics label for rpoplpush
• src/server/conn/blocking.rs:349 — BRPOPLPUSH rewrites itself to
  RPOPLPUSH src dst internally

What is missing is the part clients reach: no "RPOPLPUSH" entry in
src/command/metadata.rs and no dispatch arm beside the LMOVE arm at
src/command/mod.rs:366. This is the recurring "registered in some tables,
absent from dispatch" shape.

Scope

• RPOPLPUSH source destination ≡ LMOVE source destination RIGHT LEFT —
  implement by delegating, not by duplicating the list logic
• add the metadata.rs entry (arity 3, write, first_key 1, last_key 2, step 1,
  ACL LIST|SLOW) so COMMAND INFO/COMMAND DOCS stop lying
• wire all dispatch paths, not just the first one found — this codebase has
  more than one and a missing arm is CI-invisible
• consistency + compat-harness entries so the oracle keeps it honest

Provenance

Found while building the RESP2 null-type ground-truth table for #482, by
diffing Moon against a live redis-server 8.6.1 command-for-command. Not a
null-type bug; filed separately so #482 stays a single contract.