fix(api): make the conclusions semantic-search validation error actionable (#960)

The error raised when observer/observed are missing from a semantic-search
query states the requirement but not where the values go, so callers can't
tell whether they belong at the top level or inside `filters`. Point at the
`filters` object explicitly, show a minimal well-formed payload, and note
that both the bare and `_id`-suffixed key spellings are accepted (the code
already reads either).

Co-authored-by: hermes <hermes@local>

src/routers/conclusions.py

@@ -110,7 +110,10 @@ async def query_conclusions(
 
     if not observer or not observed:
         raise ValidationException(
-            "observer and observed must be specified for semantic search"
+            "observer and observed must be specified for semantic search. "
+            "Pass them inside the 'filters' object, e.g. "
+            '{"query": "...", "filters": {"observer": "alice", "observed": "bob"}}. '
+            "Both 'observer'/'observer_id' and 'observed'/'observed_id' are accepted."
         )
 
     with embedding_call_purpose(