ROBUSTNESS: No input validation in QCompiler.optimize() or TranspileConfig

[Manideep3969]

Issue

1. transpiler.py:149-240 — No validation for empty circuits, circuits with 0 qubits, or None inputs. Passing None causes obscure errors deep in the pipeline.

2. autotuning.py:44-70 — Users can set optimization_level=5, routing_method='invalid', or seed=-1 with no error. These fail only at Qiskit transpile time with unclear errors.

Fix

Add __post_init__ validation for TranspileConfig parameter ranges, and early validation in QCompiler.optimize() for circuit validity.