Distinguish boolean and integer queued trial parameters - #6896

Motivation

Study.enqueue_trial(..., skip_if_exists=True) currently skips True after 1 (and False after 0) because Python considers bool an int subclass. The reverse ordering is not skipped, so deduplication depends on insertion order. Boolean and integer fixed parameters should remain distinct.

Description of the changes

• Exclude mixed boolean/non-boolean fixed parameters from the duplicate match, regardless of insertion order. Leave existing numeric and same-type comparisons unchanged.
• Add regression cases for both boolean values, both insertion orders, and all seven existing storage modes; assert the two queued values retain their respective types.

RED: .venv/bin/python -m pytest tests/study_tests/test_study.py -q -k distinguishes_bool_from_int --tb=short -x -o addopts='' failed on the first case: expected 2 queued trials, got 1 ({'x': True} skipped).

GREEN:

• .venv/bin/python -m pytest tests/study_tests/test_study.py -q -k enqueue_trial_skip_existing --tb=short -o addopts='' — 147 passed.
• .venv/bin/python -m pytest tests/study_tests/test_study.py -q --tb=short -o addopts='' — 673 passed, 7 skipped (four multiprocessing fork deprecation warnings).
• .venv/bin/ruff check optuna/study/study.py tests/study_tests/test_study.py — passed.
• .venv/bin/ruff format --check optuna/study/study.py tests/study_tests/test_study.py — two files formatted.
• git diff --check — passed.

Fixes #6895. I used an LLM to assist with this PR; I reproduced the failure, selected the bool/int boundary guard, and ran the stated local checks.

Checklist

• I used an LLM while working on this PR.