fix: return owned values from generated pop_back

[tomatotomata]

Fixes the dangling-reference path reported in #830.

The generated pop_back() implementation now reads the final element through a separate @ByVal operator[] binding before resizing the container. That makes the returned JavaCPP wrapper own a copy of the value instead of aliasing the element that resize() destroys. The existing get() binding remains by-reference for normal indexed access.

I kept this scoped to resizable, long-indexed containers such as std::vector and std::deque, which are the containers that currently receive the generated pop_back() helper.

Validation:

• git diff --check
• Full Maven tests were not available because this environment has no Maven installation.

Related: #830