fix(telemetry): a search that never ran is not a decline (#3765)

`best_available_score` was added by #3670 so a bar could be judged from
what it rejected, and it arrived null on four unrelated causes: the corpus
offered nothing, the query was empty, the embedder was down, or the
DATABASE QUERY FAILED. Only the first is a measurement. The fourth is the
#2663 shape — a swallowed failure rendering as a clean zero — inside the
field added to fix an instance of the #2663 shape.

Found while trying to explain why reuse_slot returned nothing on 45 of 45
calls, and auto_inject on 153 of 161. That investigation is still open;
what it established first is that the readout could not answer it.

THE FIX IS NOT A NEW COLUMN. A call that never searched writes no row, so
every remaining null means one thing: searched, and nothing came close.
That is the convention the pre-tool arm already follows for a blank
command — "a row here would report a call that never happened and drag the
clear-rate down with phantom declines" — extended from the case a caller
can see in advance to the ones only the search knows about.

Both searches stamp `report["searched"]` FALSE before anything can return
and True only where a real result set exists, so every early return leaves
it false. It has to be the first thing done to the dict: a return added
above that line would leave the key absent.

ABSENT IS A THIRD STATE AND IT DEFAULTS TO TRUE. A caller that passes no
report cannot know, and the safe reading there is the old behaviour. Only
a real search can report False, so absent means "nobody asked" and never
"it failed" — which is also why 66 existing mocked searches across twelve
test files keep working unchanged rather than being rewritten to simulate
a flag they do not care about.

A FAILURE IS NOT MADE INVISIBLE. semantic_search_notes already logs a
WARNING on a query failure, which is where a broken search belongs: a
counter cannot say "I am broken" without a reader already trusting it.

Three tests, and the middle one is what makes them discriminate — a
blanket `return` passes the first and fails the second, because a call
that searched and came back empty is the only evidence a threshold is too
high (#3497).

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_011cPyzNnegXHr5iRMzzy5KJ

src/scribe/services/plugin_context.py

@@ -484,6 +484,7 @@ async def _reserve_slot_for_reuse(
         threshold=cfg["threshold"], limit=1, project_id=project_id,
         is_task=None, results=reuse,
         best_available=_rep.get("best_available_score"),
+        searched=bool(_rep.get("searched", True)),
         duration_ms=(time.perf_counter() - _t0) * 1000.0,
     )
     # Verify the kind rather than trusting the query that asked for it, and
@@ -552,6 +553,7 @@ async def build_autoinject_hint(
         threshold=cfg["threshold"], limit=cfg["top_k"],
         project_id=(project_id or None), is_task=None, results=hits,
         best_available=_rep_ai.get("best_available_score"),
+        searched=bool(_rep_ai.get("searched", True)),
         duration_ms=(time.perf_counter() - t0) * 1000.0,
     )
     if not hits:
@@ -1046,6 +1048,7 @@ async def build_write_path_hint(
             best_available=(
                 None if withheld_here else _rep_wp.get("best_available_score")
             ),
+            searched=bool(_rep_wp.get("searched", True)),
             duration_ms=(time.perf_counter() - t0) * 1000.0,
         )
         if hits:
@@ -1342,6 +1345,7 @@ async def build_write_path_hint(
             project_id=project_id,
             is_task=None, results=fresh, duration_ms=rule_ms,
             best_available=_rep_wpr.get("best_available_score"),
+            searched=bool(_rep_wpr.get("searched", True)),
             # What the ranker found and this session had already been told.
             # Without it a zero row cannot say whether the bar was too high or
             # the reader was simply ahead of it — and only the first is a
@@ -1449,6 +1453,7 @@ async def build_tool_rule_hint(
             project_id=project_id,
             is_task=None, results=fresh, duration_ms=duration_ms,
             best_available=_rep_ptr.get("best_available_score"),
+            searched=bool(_rep_ptr.get("searched", True)),
             # See the sibling arm. It matters more here: this arm fires on every
             # Bash call, so a long session excludes its way to an all-zero row
             # and the threshold looks wrong when nothing about it is.