fix(transaction): make ContextVarsTransactionContextPropagator use per-instance storage

Signed-off-by: marcozabel <marco.zabel@dynatrace.com>

Author: marcozabel

openfeature/transaction_context/context_var_transaction_context_propagator.py

@@ -7,9 +7,10 @@
 
 
 class ContextVarsTransactionContextPropagator(TransactionContextPropagator):
-    _transaction_context_var: ContextVar[EvaluationContext | None] = ContextVar(
-        "transaction_context", default=None
-    )
+    def __init__(self) -> None:
+        self._transaction_context_var: ContextVar[EvaluationContext | None] = (
+            ContextVar(f"transaction_context_{id(self)}", default=None)
+        )
 
     def get_transaction_context(self) -> EvaluationContext:
         context = self._transaction_context_var.get()