test(mongodb): keep the integration helper in step with the Atlas-legal name

engine_index() hardcoded '{TEST_INDEX}:{engine_name}', duplicating the
production rule rather than calling it -- which is both why it broke here and
part of why the colon survived to a real Atlas run in the first place.

It has to duplicate: the engine lives in a bin target, so an integration test
cannot import derive_search_index_name. The comment now says so, and says why
the separator must not be 'fixed' back to ':' -- mongodb-atlas-local accepts a
colon that hosted Atlas rejects, so this suite can never be the thing that
catches it. The unit test search_index_names_are_atlas_legal is.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

Author: fcostaoliveira

tests/integration_mongodb.rs

@@ -128,8 +128,20 @@ fn engine_collection(engine_name: &str) -> String {
 
 /// The search index the ENGINE builds for config `engine_name` when the run is
 /// given `MONGODB_INDEX_NAME=TEST_INDEX` (#306).
+///
+/// The separator is `_`, mirroring `derive_search_index_name`: MongoDB Atlas
+/// rejects a `:` in a search index name with
+/// `BadValue: invalid index name`, even though the
+/// `mongodb/mongodb-atlas-local` image these tests run against accepts it. That
+/// permissiveness gap is why the colon survived to a real Atlas run, so keep
+/// this helper in step with production and do not "fix" it back to `:`.
+///
+/// It has to duplicate the production rule rather than call it: the engine lives
+/// in a bin target, so an integration test cannot import `derive_search_index_name`.
+/// The unit test `search_index_names_are_atlas_legal` is what actually guards the
+/// character set.
 fn engine_index(engine_name: &str) -> String {
-    format!("{TEST_INDEX}:{engine_name}")
+    format!("{TEST_INDEX}_{engine_name}")
 }
 
 /// Count documents in a specific collection of the test database, server-side.