fix(bindings): accept a default TileView offset

[high-cloud]

Summary

Make the nanobind TileView constructor handle its default/optional start offset without an ambiguous conversion.

Current PyPTO main exposes an ExprPtr constructor with None as the default argument. With the wheel build used by pypto-serving NPU CI, evaluating ir.TileView() fails in _implicit_tile_view_defaults with an incompatible-constructor TypeError. Model compilation then aborts and the Serving worker reports zero KV cache blocks.

• Bind the true C++ default constructor explicitly.
• Accept a Python object for start_offset, map None to a null ExprPtr, and otherwise cast it to ExprPtr before placement construction.
• Preserve the existing integer-shape constructor and all public defaults.

Observed downstream failure: hw-native-sys/pypto-serving#180 unit-tests. This PR is independent of the full-rank dispatch work in #2490.

Verification

• Rebuilt the PyPTO extension with this patch.
• tests/ut/language/parser/test_type_resolver.py
• tests/ut/ir/core/test_tile_view_equality.py
• tests/ut/ir/test_tensor_view_semantics.py

Result: 243 passed.

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