perf: add reference_internal policy for const-ref returns

[jashshah999]

Summary

• Methods returning const T& now emit py::return_value_policy::reference_internal in the generated pybind11 bindings
• The generated lambda uses -> const auto& trailing return type to preserve the reference semantics
• This avoids unnecessary copies when Python accesses C++ members by const reference, keeping the returned reference tied to the parent object's lifetime

Motivation

Without this policy, pybind11 defaults to copy for returned references from lambdas (since the lambda return type is deduced as a value). This causes:

1. Unnecessary deep copies of potentially large objects (matrices, vectors)
2. Surprising semantics where modifying the "reference" in Python doesn't affect the C++ object

With reference_internal, pybind11 returns a reference and ties the Python reference's lifetime to the parent object, matching C++ semantics.

Changes

• gtwrap/pybind_wrapper.py: Detect is_ref on the return type and emit -> const auto& + py::return_value_policy::reference_internal for instance methods
• tests/expected/python/class_pybind.cpp: Updated expected output for return_vector2 and return_matrix2 const-ref overloads

Test plan

• All existing tests pass (pytest tests/test_pybind_wrapper.py - 9/9 passing)
• Verify with a downstream project (e.g. GTSAM) that const-ref accessors no longer copy