fix(handler): optimise snapshot hanlding (#8652)

# Which Problems Are Solved

There are cases where not all statements of multiExec are succeed. This
leads to inconsistent states. One example is [LDAP
IDPs](https://github.com/zitadel/zitadel/issues/7959).

If statements get executed only partially this can lead to inconsistent
states or even break projections for objects which might not were
correctly created in a sub table.

This behaviour is possible because we use
[`SAVEPOINTS`](https://www.postgresql.org/docs/current/sql-savepoint.html)
during each statement of a multiExec.

# How the Problems Are Solved

SAVEPOINTS are only created at the beginning of an exec function not
during every execution like before. Additionally `RELEASE` or `ROLLBACK`
of `SAVEPOINTS` are only used when needed.

# Additional Changes

- refactor some unused parameters

# Additional Context

- closes https://github.com/zitadel/zitadel/issues/7959

internal/query/projection/executer_test.go

@@ -25,7 +25,7 @@ type execution struct {
 type anyArg struct{}
 
 func (e *testExecuter) Exec(stmt string, args ...interface{}) (sql.Result, error) {
-	if stmt == "SAVEPOINT stmt_exec" || stmt == "RELEASE SAVEPOINT stmt_exec" {
+	if stmt == "SAVEPOINT exec_stmt" {
 		return nil, nil
 	}