test(integration): move contention tests to multitenant module

Contention tests must run in the multitenant phase where different
tenant keysets compete for the shared ZerokmsClient mutexes — the
actual customer scenario. Tests now use SET CIPHERSTASH.KEYSET_ID
with 3 tenant keysets, separate connection setup from timing, and
increase volume to 50 inserts per tenant to surface contention.

Author: tobyhede

packages/cipherstash-proxy-integration/src/contention.rs

@@ -1,5 +1,4 @@
 mod common;
-mod contention;
 mod decrypt;
 mod diagnostics;
 mod disable_mapping;