fix(lint): fix false negative in noMisleadingReturnType for union annotations (#10052)

Author: minseong0324

.changeset/lazy-days-know.md

@@ -0,0 +1,12 @@
+---
+"@biomejs/biome": patch
+---
+
+Improved [`noMisleadingReturnType`](https://biomejs.dev/linter/rules/no-misleading-return-type/): it now flags union annotations whose extra variants are never returned, and suggests the narrower type (e.g. `string | null` → `string`).
+
+These functions are now reported because `null` and `number` are included in the return annotations but never returned:
+
+```ts
+function getUser(): string | null { return "hello"; }   // null is never returned
+function getCode(): string | number { return "hello"; } // number is never returned
+```