v2.6.0: adverse-event summary tier, always-on search criteria echo, channel parity

adverse-event summary tier, always-on search criteria echo, channel parity

Surfaces top adverse events in summary mode, fixes format() truncation in full mode,
and closes the content[]/structuredContent channel gap in both get_study_results
and get_study_record. Also adds query-param parity to get_study_count and declares
the enum_invalid error contract on search and count tools.

Added:

• enum_invalid error contract on search_studies and get_study_count (#57)
• sentinelFilterActive in search_studies searchCriteria enrichment (#58)
• locationQuery, titleQuery, outcomeQuery on get_study_count (#59)
• topEvents array in get_study_results summary mode — top N adverse events by numAffected, capped at 20 (#61)

Changed:

• search_studies searchCriteria now echoes on every response, not only on empty results (#58)
• get_study_results full mode: row caps removed from format() — renders all adverse-event and baseline rows (#63)

Fixed:

• Channel parity: content[] and structuredContent now carry identical data (#63)
• get_study_record structuredContent no longer embeds full resultsSection (was absent from content[]); compact resultsSummary returned instead

Breaking:

• get_study_record structuredContent.study.resultsSection removed; use clinicaltrials_get_study_results or clinicaltrials://{nctId}

491 tests pass; bun run devcheck clean.