refactor: align deCODE harmonisation to shared config pattern and common/processing utilities

[project-defiant]

✨ Context

This PR follows up on the unification of harmonisation efforts in gentropy. Here we focus on deCODE summary statistics harmonisation.

🛠 What does this PR implement

• Adopt common/processing helpers in deCODESummaryStatistics.from_source: replace private _infer_allele_frequency with shared infer_allele_frequency_from_maf; use flag_equal_alleles and flag_non_atgc_alleles in place of inline predicates
• Collapse remove_star_alleles and remove_multiallelics into verify_atgc in both deCODESummaryStatisticsHarmonisationConfig and the step init — the ATGC predicate already excludes both star and multiallelic markers
• Remove inline Python defaults from deCODESummaryStatisticsHarmonisationStep init; all defaults are now declared once in the Hydra config dataclass
• Rename deCODEHarmonisationConfig fields: min_mac → min_allele_count_threshold, min_sample_size → sample_size_threshold; add perform_* toggle booleans
• Move DECODE_SCHEMA to module-level constant; add ConfigDict(extra="forbid") to deCODEHarmonisationConfig
• Make EFOMapping.annotate_study_index generic (TypeVar S bound=StudyIndex) so it returns the concrete subclass type rather than base StudyIndex
• Fix SessionConfig s3_configuration/gcs_configuration defaults to use field(default_factory=...) for Hydra structured config compatibility

🙈 Missing

🚦 Before submitting

• Do these changes cover one single feature (one change at a time)?
• Did you read the contributor guideline?
• Did you make sure to update the documentation with your changes?
• Did you make sure there is no commented out code in this PR?
• Did you follow conventional commits standards in PR title and commit messages?
• Did you make sure the branch is up-to-date with the dev branch?
• Did you write any new necessary tests?
• Did you make sure the changes pass local tests (make test)?
• Did you make sure the changes pass pre-commit rules (e.g uv run pre-commit run --all-files)?