Fix block particle filter lint issues

[FlorianPfaff]

Summary

• remove the unused block particle filter backend import
• make the block particle mixin contract explicit for type checkers
• use a type-checker-friendly parent set_particles lookup and add a default n_particles helper

Verification

• python -m pylint src/pyrecest/filters/block_particle_filter.py
• python -m flake8 src/pyrecest/filters/block_particle_filter.py
• PYTHONPATH=src python -m mypy --ignore-missing-imports --follow-imports=skip src/pyrecest/filters/block_particle_filter.py
• PYTHONPATH=src python -m pytest tests/filters/test_euclidean_boxed_particle_filter.py tests/filters/test_so3_product_particle_filter.py tests/filters/test_partitioned_so3_product_particle_filter.py

[github-actions]

✅MegaLinter analysis: Success

Descriptor	Linter	Files	Fixed	Errors	Warnings	Elapsed time
✅ COPYPASTE	jscpd	yes		no	no	18.82s
✅ JSON	prettier	2	0	0	0	0.44s
✅ JSON	v8r	2		0	0	2.39s
✅ MARKDOWN	markdownlint	28	0	0	0	1.39s
✅ MARKDOWN	markdown-table-formatter	28	0	0	0	0.4s
✅ PYTHON	bandit	522		0	0	8.57s
✅ PYTHON	black	522	1	0	0	13.48s
✅ PYTHON	flake8	522		0	0	4.88s
✅ PYTHON	isort	522	1	0	0	0.98s
✅ PYTHON	mypy	522		0	0	6.53s
✅ PYTHON	pylint	522		0	0	116.26s
✅ PYTHON	ruff	522	1	0	0	0.09s
✅ REPOSITORY	checkov	yes		no	no	23.39s
✅ REPOSITORY	gitleaks	yes		no	no	13.65s
✅ REPOSITORY	git_diff	yes		no	no	0.11s
✅ REPOSITORY	secretlint	yes		no	no	9.62s
✅ REPOSITORY	syft	yes		no	no	3.24s
✅ REPOSITORY	trivy-sbom	yes		no	no	3.65s
✅ REPOSITORY	trufflehog	yes		no	no	21.62s
✅ YAML	prettier	5	0	0	0	0.54s
✅ YAML	v8r	5		0	0	5.2s
✅ YAML	yamllint	5		0	0	0.54s

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[github-actions]

Test Results

    12 files  ±0      12 suites  ±0   2h 29m 23s ⏱️ + 4m 10s
 1 545 tests ±0   1 545 ✅ ±0      0 💤 ±0  0 ❌ ±0 
19 000 runs  ±0  15 216 ✅ ±0  3 784 💤 ±0  0 ❌ ±0 

Results for commit 68dd62a. ± Comparison against base commit a6a093e.