Bare expression statements crash with AttributeError instead of raising ValidationError #388

Limitation

An expression followed by a semicolon is a syntactically valid OpenQASM 3 statement — the
classical types chapter uses this form throughout to
illustrate operator results, for example under
Integers and
Angles.

pyqasm reaches these statements and dereferences .name on the expression node. For any
non-Identifier expression this raises an AttributeError that escapes the public API. The
same dereference fails for an undeclared gate applied to zero qubits.

Example QASM failure

Straight from the spec's Integers section:

import pyqasm

m = pyqasm.loads('''
OPENQASM 3.0;
int[32] a = 2;
int[32] b = 3;

a * b;   // 6
b / a;   // 1
b % a;   // 1
''')
m.validate()

AttributeError: 'BinaryExpression' object has no attribute 'name'

The spec's Angles section fails identically:

OPENQASM 3.0;
angle[4] a = 7 * (pi / 8);
angle[4] b = pi / 8;
a + b;
// AttributeError: 'BinaryExpression' object has no attribute 'name'

And the gate-application example in
Applying gates:

OPENQASM 3.0;
g1 q1;
// AttributeError: 'str' object has no attribute 'name'

Change Requested

1. Never surface an AttributeError from a statement pyqasm does not handle. Every
   unhandled statement or expression form must raise ValidationError carrying the source
   span.
2. Decide and document the semantics of a bare expression statement. It has no effect on the
   quantum program, so the reasonable options are to evaluate and discard it, or to reject it
   with a clear message such as "Expression statement has no effect". Either is acceptable;
   the current crash is not.
3. An undefined gate identifier must report the undefined name, not fail on an attribute
   lookup.

Implementation Details

• The root cause is the visitor's dispatch table in src/pyqasm/visitor.py:193:

  qasm3_ast.ExpressionStatement: lambda x: self._visit_function_call(x.expression),

  Every expression statement is assumed to be a function call, so _visit_function_call
  dereferences x.expression.name — which exists on an Identifier but not on a
  BinaryExpression, a UnaryExpression, an IndexExpression, or a literal.

• The fix is to branch on the expression type before dispatching: route FunctionCall to
  _visit_function_call and give every other form the chosen behaviour from point 2 above.
  The generic fallback at visitor.py:3492 already raises the correct error shape and can be
  reused.

• The 'str' object has no attribute 'name' variant comes from the gate-operation path, where
  the operation name has already been reduced to a string before the undefined-gate check
  runs. Reorder so the lookup failure is reported first.

• Add a defensive try/except (AttributeError, TypeError) around the visitor's top-level
  dispatch that re-raises as ValidationError with the statement span. This converts any
  remaining internal slips in the tree into the documented error type — a cheap safety net
  beyond this specific fix.

• Tests: tests/qasm3/test_validation.py — assert ValidationError (not AttributeError)
  for a bare binary expression, a bare function call, a bare identifier, and an undefined gate
  with zero and with one qubit operand.