Honor explicit pop defaults without creating missing values

[xiehuanyi]

With default_box=True, Box().pop("missing", fallback) ignores the explicit fallback and calls the configured default factory. If default_box_create_on_get=False, it can instead raise BoxKeyError while deleting the generated but unstored value.

Use the existing default-suppression lookup for pop calls with an explicit fallback, and propagate it through dotted Box and BoxList paths. Return the original fallback object without calling the factory or creating missing values. Add regressions for both creation modes, scalar/collection fallbacks, nested mapping/list paths, and existing camel-case keys. Include the changelog and contributor entry requested by the guide.

Validation on Linux/Python 3.14: 11 of the 12 new cases fail on unchanged develop; all 171 tests pass in pure Python and again after compiling the runtime with Cython, with the declared optional test dependencies installed. Black with the project configuration, Pyflakes, and mypy (8 source files) pass.

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