Fix a client's coverage when it is first entered

`_resolve_scope` returned a scope without keeping it, so a client re-entered
after its configuration was edited covered whatever the configuration then said.
Resolving once is what the rest of the design rests on: the scope is what names
results, citations and errors.

A configuration is still free to change before first entry.

haiku_rag_slim/haiku/rag/client/__init__.py

@@ -314,6 +314,7 @@ class HaikuRAG:
         )
         if self._requested_sources is not None and self._requested_db_path is None:
             scope = scope.select(self._requested_sources)
+        self._scope = scope
         return scope
 
     async def __aenter__(self):

tests/multi_db/test_lifecycle.py

@@ -94,6 +94,23 @@ class TestOpeningDatabases:
 
         assert not alpha.store.db.is_open()
 
+    @pytest.mark.asyncio
+    async def test_a_client_keeps_the_databases_it_first_covered(self, tmp_path):
+        """Resolution happens once, so a configuration edited afterwards does not
+        change what an already-entered client covers."""
+        config = _config(tmp_path, ["alpha", "beta"])
+        await _seed(config, "alpha", ["alpha document about cats"])
+        await _seed(config, "beta", ["beta document about cats"])
+
+        rag = HaikuRAG(config=config)
+        async with rag:
+            assert rag.source_names == ("alpha", "beta")
+
+        config.lancedb.databases = {"gamma": str(tmp_path / "gamma.lancedb")}
+
+        async with rag:
+            assert rag.source_names == ("alpha", "beta")
+
     @pytest.mark.asyncio
     async def test_a_failing_read_leaves_no_sibling_reading(
         self, tmp_path, monkeypatch