LCORE-1572: raise instead of assert on the drained compaction result

apply_compaction_blocking asserted that the generator yielded a result. Under
python -O asserts are stripped, so the guard would vanish and a None result
could propagate to callers. Replace it with an explicit None check that raises
RuntimeError.

Clears a GitHub code-scanning (CodeQL) "use of assert" finding. The repository's
Bandit configuration skips B101, so this only surfaced via code scanning, not
the Bandit CI job.

Author: max-svistunov

src/utils/conversation_compaction.py

@@ -375,7 +375,8 @@ async def apply_compaction_blocking(
     ):
         if isinstance(item, CompactionResult):
             result = item
-    assert result is not None  # the generator always yields exactly one result
+    if result is None:  # pragma: no cover - the generator always yields one result
+        raise RuntimeError("apply_compaction did not yield a CompactionResult")
     return result