Require confirmed cache usage in strict investigation tests

config/llm.gemini-cache.example.toml

@@ -25,6 +25,10 @@ cache_ttl_seconds = 300
 # A character heuristic, not a tokenizer or a format limit. Provider minimums
 # still apply; rejected/too-small cache requests fall back to the full input.
 cache_min_prefix_chars = 16000
+# For an explicit cache test, require confirmed hits in investigation calls:
+# cache_required = true
+# cache_required_prompt_prefix = "investigation_project_"
+# Omit the prefix to require caching in ALL phases (small prefixes may fail).
 
 # Optional estimates in USD per million tokens; verify your model/tariff/date.
 # These illustrative standard-tier rates are from 2026-10-16, valid through