Accept list pairs inside tuple constructor inputs

[Eric3-jp]

Dict((['a', 1], ['b', 2])) raises TypeError instead of producing {'a': 1, 'b': 2}. A tuple containing just one list pair raises IndexError. The constructor mistakes the outer tuple for the single-pair shortcut because it only recognizes a tuple, rather than a list, as the first nested pair. The same pairs already work in a list or generator, and tuple-of-pairs construction was introduced in #37.

Route tuples beginning with a list pair through the existing iterable-of-pairs path. This retains the single-pair tuple shortcut and the existing recursive conversion of nested dictionaries.

Add regression coverage for multiple list pairs, a single list pair with a nested dictionary, mixed list/tuple pairs with a tuple key, and the existing tuple-of-tuples form. Each case runs for both Dict and its test subclass.

Validation:

• Before the source change, the eight new regression/control cases had six errors and two passes.
• python -m unittest -q test_addict: all 136 tests passed on Windows with CPython 3.14.0 and Ubuntu 24.04 with CPython 3.11.16.
• python -m compileall -q addict test_addict.py and git diff --check: passed.

The older Python versions in the CI matrix were not run locally.

Developed with assistance from OpenAI Codex.