refactor: consolidate cluster slot checks in connection.rs

[kacy]

summary

extracts a single cluster_slot_check() function that pattern-matches the command to determine its routing key(s), validates slot ownership and crossslot constraints, and returns the redirect/error frame if needed. this replaces the per-command inline checks with one call at the top of execute().

• removed: ~130 lines of repeated 3-line (check_cluster_slot) and 7-line (check_crossslot + check_cluster_slot) blocks scattered across ~39 match arms
• added: ~60 lines in one focused function with clear grouping (single-key, multi-key, rename, mset, passthrough)
• net reduction of ~112 lines

what was tested

• cargo build -p ember-server — compiles clean
• cargo clippy --workspace -- -D warnings — no warnings
• cargo test --workspace — all tests pass (behavior unchanged)

design considerations

the new function uses a single match on &Command to extract the routing key(s), grouping commands into four categories:

1. single-key: 37 variants bound via ref key — delegates to cluster.check_slot()
2. multi-key (DEL, UNLINK, EXISTS, MGET): crossslot check on all keys, then slot check on the first
3. rename: crossslot check on [key, newkey], then slot check on key
4. mset: extracts keys from pairs, same crossslot + slot pattern
5. wildcard: everything else (PING, INFO, cluster commands, pubsub, etc.) returns None — no slot routing needed

the function is called once at the top of execute(), before the main dispatch match. commands that don't need slot routing fall through the wildcard arm with zero overhead (just a pattern match, no async work).