Fix marginal counts for mixed-width hex Counts

[snoopuppy582]

Fixes #16190.

Summary

This updates count marginalization to infer the classical-bit width from the longest cleaned count key, then treats shorter keys as left-zero-padded while marginalizing. This fixes Counts objects created from hexadecimal input without memory_slots, where the stored bitstrings can have mixed widths such as "0" and "11".

The same width handling is applied to the accelerated marginal-distribution path so higher-bit indices do not panic on mixed-width keys.

Details and comments

• marginal_counts(Counts({"0x0": 50, "0x3": 30}), [0]) now returns {"0": 50, "1": 30}.
• marginal_distribution(..., [1]) now handles the same input instead of panicking in the Rust helper.
• The no-indices path still only strips spaces/underscores and does not pad keys.

Testing

• python -X utf8 -m unittest test.python.result.test_counts.TestCounts.test_marginal_counts_mixed_width_hex_without_memory_slots test.python.result.test_counts.TestCounts.test_marginal_distribution_mixed_width_hex_without_memory_slots
• python -X utf8 -m unittest test.python.result.test_counts
• python -X utf8 -m unittest test.python.result.test_result.TestResultOperations.test_marginal_counts test.python.result.test_result.TestResultOperations.test_marginal_distribution test.python.result.test_result.TestResultOperations.test_marginal_counts_result test.python.result.test_result.TestResultOperations.test_marginal_counts_with_dict test.python.result.test_result.TestResultOperationsFailed.test_marginal_counts_no_cregs
• python -X utf8 -m unittest discover -s test/python/result -p 'test_*.py'
• cargo test -p qiskit-accelerate --lib
• cargo fmt --check
• black --check qiskit/result/utils.py test/python/result/test_counts.py
• ruff check qiskit/result/utils.py test/python/result/test_counts.py
• reno lint

AI/LLM disclosure

• I didn't use LLM tooling, or only used it privately.
• I used OpenAI Codex (GPT-5) to help write this PR description.
• I used OpenAI Codex (GPT-5) to help investigate the issue and draft code/test changes; I reviewed and verified the final diff locally.

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