[Fix][Relax] Preserve symbolic shapes during constant folding - #20431

Motivation

For valid symbolic shapes, the shape_to_tensor fallback in FoldConstant can encounter a PrimExpr that is not an IntImmNode. Dereferencing it as an integer constant is unsafe. In the valid shape_of -> shape_to_tensor -> tensor_to_shape -> reshape path, reshape legalization also assumes a shape Var is bound directly to a ShapeExpr, but it may be bound to a call.

Related to #20260.

Changes

• Fold shape_to_tensor through the C++ fallback only if each dimension is an int64 IntImmNode with a value representable as int64_t; otherwise retain the call.
• Preserve reshape when its target shape Var is not bound to a ShapeExpr, rather than asserting during legalization.
• Add regression tests for symbolic and mixed shapes, static folding through the fallback, and a valid dynamic reshape. The fallback tests temporarily remove and restore FLegalize to exercise the C++ path.

Validation

• The complete tests/python/relax/test_transform_fold_constant.py file passes: 23 tests on the rebased source.
• The original valid dynamic-reshape reproducer completes FoldConstant without an exception.
• Both the default and cpu_generic LLVM/CPU pipelines build and execute with input lengths 3 and 7; outputs match the inputs.
• Pre-commit checks pass on the three changed files.

Scope

Passing the Tensor result of shape_to_tensor directly to reshape without tensor_to_shape remains outside this change. In the BlockBuilder reproducer, that input is rejected before FoldConstant because reshape expects a Shape-typed operand. This PR does not relax that contract.