expand useLiveQuery callback to support conditional queries and additional return types

Author: samwillis

.changeset/spotty-dogs-fry.md

@@ -0,0 +1,46 @@
+---
+"@tanstack/react-db": patch
+---
+
+Expand `useLiveQuery` callback to support conditional queries and additional return types, enabling the ability to temporarily disable the query.
+
+**New Features:**
+- Callback can now return `undefined` or `null` to temporarily disable the query
+- Callback can return a pre-created `Collection` instance to use it directly
+- Callback can return a `LiveQueryCollectionConfig` object for advanced configuration
+- When disabled (returning `undefined`/`null`), the hook returns a specific idle state
+
+**Usage Examples:**
+
+```ts
+// Conditional queries - disable when not ready
+const enabled = useState(false)
+const { data, state, isIdle } = useLiveQuery((q) => {
+  if (!enabled) return undefined  // Disables the query
+  return q.from({ users }).where(...)
+}, [enabled])
+
+/**
+ * When disabled, returns:
+ * { 
+ *   state: undefined,
+ *   data: undefined,
+ *   isIdle: true,
+ *   ...
+ * }
+ */
+
+// Return pre-created Collection
+const { data } = useLiveQuery((q) => {
+  if (usePrebuilt) return myCollection  // Use existing collection
+  return q.from({ items }).select(...)
+}, [usePrebuilt])
+
+// Return LiveQueryCollectionConfig
+const { data } = useLiveQuery((q) => {
+  return {
+    query: q.from({ items }).select(...),
+    id: `my-collection`,
+  }
+})
+```