Changed the method ExprOperator::GetSignature to return a shared-ptr

Motivation: The primary goal of this change is to provide efficient access to the operator’s aux_policy without incurring the cost of copying the full signature.

This enables the use of `op.signature.aux_policy` for `ExprView` customisation. It also allows moving the `Validate*Count()` utilities to the `ExprOperator` base class, which was previously avoided because of the performance penalty of signature copying.

---

Benchmark results for future reference:

```python
from arolla import arolla

_1 = arolla.int32(1)
_2 = arolla.int32(2)
op = arolla.M.math.add

%timeit arolla.abc.get_operator_signature(op)  # 356ns -> 313ns
%timeit arolla.abc.aux_bind_op(op, _1, _2)     # 1.05us -> 1.02us
%timeit arolla.abc.aux_eval_op(op, _1, _2)     # 442ns -> 405ns
```

PiperOrigin-RevId: 858086299
Change-Id: I764a2d50e6db68367cd0d4c94c0a4c8396eb1398

Author: apronchenkov

py/koladata/expr/py_expr_eval.cc

@@ -179,23 +179,23 @@ PyObject* absl_nullable PyEvalOp(PyObject* /*self*/, PyObject** py_args,
   ASSIGN_OR_RETURN(auto signature, op->GetSignature(), SetPyErrFromStatus(_));
   std::vector<QValueOrExpr> bound_args;
   AuxBindingPolicyPtr policy_implementation;
-  if (!AuxBindArguments(signature, py_args + 1,
+  if (!AuxBindArguments(*signature, py_args + 1,
                         (nargs - 1) | PY_VECTORCALL_ARGUMENTS_OFFSET,
                         py_kwnames, &bound_args, &policy_implementation)) {
     return nullptr;
   }
 
   // Generate `input_qvalues`.
   const auto param_name = [&signature](size_t i) -> std::string {
-    if (!HasVariadicParameter(signature)) {
-      DCHECK_LT(i, signature.parameters.size());
-      return signature.parameters[i].name;
+    if (!HasVariadicParameter(*signature)) {
+      DCHECK_LT(i, signature->parameters.size());
+      return signature->parameters[i].name;
     }
-    if (i + 1 < signature.parameters.size()) {
-      return signature.parameters[i].name;
+    if (i + 1 < signature->parameters.size()) {
+      return signature->parameters[i].name;
     }
-    return absl::StrCat(signature.parameters.back().name, "[",
-                        i - signature.parameters.size() + 1, "]");
+    return absl::StrCat(signature->parameters.back().name, "[",
+                        i - signature->parameters.size() + 1, "]");
   };
 
   std::vector<TypedRef> input_qvalues;

py/koladata/operators/py_unified_binding_policy.cc

@@ -734,7 +734,7 @@ class UnifiedBindingPolicy : public AuxBindingPolicy {
   // (See the base class.)
   ExprNodePtr absl_nullable MakeLiteral(TypedValue&& value) const final {
     ASSIGN_OR_RETURN(auto result, koladata::expr::MakeLiteral(std::move(value)),
-                     (SetPyErrFromStatus(_), nullptr));
+                     SetPyErrFromStatus(_));
     return result;
   }
 };