Make per-context behavior clearer + expose Contexts in cohort summaries

[sar9ho]

Overview

Closes #143

Description of changes

• Changed the model evaluation title to say “Per Context” (instead of “Per Encounter”) when Combine scores/per_context_id is turned on
• Updated the docstrings so it’s clear that Combine scores aggregates by (entity_id, context_id) from usage_config.yml
• Updated cohort summary tables to optionally show a Contexts count (grouped by (entity_id, context_id)) when context_id is configured, while keeping Entities as the count grouped by entity_id only
• Added/updated tests to cover the regression + the new cohort summary behavior

Author Checklist

• Linting passes; run early with pre-commit hook.
• Tests added for new code and issue being fixed.
• Added type annotations and full numpy-style docstrings for new methods.
• Draft your news fragment in new changelog/ISSUE.TYPE.rst files; see changelog/README.md.