QueryContainerVariableRetentionTest

Proves the legacy QueryContainerBuilder variable map holds values by strong reference: values written through the builder must remain readable after GC pressure instead of being evicted like weak references.

QueryContainerBuilder stores variables in a WeakHashMap; entries survive only while their keys stay strongly reachable. This test writes variables with both interned literals and runtime-generated keys, forces garbage collection, and asserts the values are still readable, which is the behavior the historical artifact relied on.