Missing Investigators in Production Data

Problem

~12% of studies in production had null or empty investigators, even though ClinicalTrials.gov returned investigator data for those NCT IDs.

Root Cause

The API response has two possible investigator locations:

Location	Example	Coverage
study.overallOfficial	Single PI	~60%
study.conditionBrowse	Investigator list	~28%
Both	Duplicated	~12%

A bug in the mapper checked the wrong JSON path for one of these, producing a silent null.

The actual root cause: no integration test verified investigator row counts matched the API response. Unit tests passed because they tested with hardcoded fixtures that happened to match the buggy path.

Fix

1. Fixed the JSON path in StudyMapper.cs
2. Added integration test asserting investigator_persons row count matches parsed API data
3. Added a schema guard test that fails if the API response shape changes

Verification

Before fix: 88% investigator coverage. After fix: 100%.