fix(optimizer): prevent Shape/Gather from folding to constants when shape has negative (unknown) dims

[Copilot]

FoldConstantsPass incorrectly folds Shape→Gather→Concat→Reshape patterns when the input tensor has unknown dimensions stored as integer -1 in the IR (as opposed to ir.SymbolicDim). Since -1 is an int, the all-integer check passes, causing Shape to fold into Constant(value_ints=[-1, 512, 1, ...]). Gather then returns a constant -1, which propagates into the Reshape shape — producing an invalid shape like [-1, -1] (ONNX allows at most one -1 in a Reshape shape).

Changes

• Shape handler: add non-negativity guard — only fold to a constant when all shape dims are known positive integers:

  # before
  if all(isinstance(d, int) for d in shape_slice):
  # after
  if all(isinstance(d, int) and d >= 0 for d in shape_slice):

• Gather handler: same guard on the symbolic-shape gather path, preventing folding when any gathered dimension value is negative.
• Regression test: test_shape_gather_concat_reshape_with_neg1_dim — verifies that Shape is preserved and Reshape never receives multiple -1 values when the input shape contains -1 integer dims.

The root cause: ONNX IR deserializes unknown dims (dim_value=-1 in the proto) as integer -1, not ir.SymbolicDim. The existing SymbolicDim path was safe; the -1-as-int path was not.