feat(storage): share sql/kv instances and add upsert support (#4140)

A few changes to the storage layer to ensure we reduce unnecessary
contention arising out of our design choices (and letting the database
layer do its correct thing):

- SQL stores now share a single `SqlAlchemySqlStoreImpl` per backend,
and `kvstore_impl` caches instances per `(backend, namespace)`. This
avoids spawning multiple SQLite connections for the same file, reducing
lock contention and aligning the cache story for all backends.

- Added an async upsert API (with SQLite/Postgres dialect inserts) and
routed it through `AuthorizedSqlStore`, then switched conversations and
responses to call it. Using native `ON CONFLICT DO UPDATE` eliminates
the insert-then-update retry window that previously caused long WAL lock
retries.

### Test Plan

Existing tests, added a unit test for `upsert()`

tests/unit/utils/sqlstore/test_sqlstore.py

@@ -9,7 +9,7 @@ from tempfile import TemporaryDirectory
 
 import pytest
 
-from llama_stack.providers.utils.sqlstore.api import ColumnType
+from llama_stack.providers.utils.sqlstore.api import ColumnDefinition, ColumnType
 from llama_stack.providers.utils.sqlstore.sqlalchemy_sqlstore import SqlAlchemySqlStoreImpl
 from llama_stack.providers.utils.sqlstore.sqlstore import SqliteSqlStoreConfig
 
@@ -65,6 +65,38 @@ async def test_sqlite_sqlstore():
         assert result.has_more is False
 
 
+async def test_sqlstore_upsert_support():
+    with TemporaryDirectory() as tmp_dir:
+        db_path = tmp_dir + "/upsert.db"
+        store = SqlAlchemySqlStoreImpl(SqliteSqlStoreConfig(db_path=db_path))
+
+        await store.create_table(
+            "items",
+            {
+                "id": ColumnDefinition(type=ColumnType.STRING, primary_key=True),
+                "value": ColumnType.STRING,
+                "updated_at": ColumnType.INTEGER,
+            },
+        )
+
+        await store.upsert(
+            table="items",
+            data={"id": "item_1", "value": "first", "updated_at": 1},
+            conflict_columns=["id"],
+        )
+        row = await store.fetch_one("items", {"id": "item_1"})
+        assert row == {"id": "item_1", "value": "first", "updated_at": 1}
+
+        await store.upsert(
+            table="items",
+            data={"id": "item_1", "value": "second", "updated_at": 2},
+            conflict_columns=["id"],
+            update_columns=["value", "updated_at"],
+        )
+        row = await store.fetch_one("items", {"id": "item_1"})
+        assert row == {"id": "item_1", "value": "second", "updated_at": 2}
+
+
 async def test_sqlstore_pagination_basic():
     """Test basic pagination functionality at the SQL store level."""
     with TemporaryDirectory() as tmp_dir: