Lower AI search query cap from 2000 to 500 chars (#61481)

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: PickHub

src/search/lib/ai-search-constants.ts

@@ -1,4 +1,4 @@
 // Maximum query length (chars) we will forward to cse-copilot. Larger
 // payloads are almost always pasted docs pages or unrelated content
 // and either time out or return no-answer. See github/cse-copilot#1214.
-export const MAX_QUERY_LENGTH = 2000
+export const MAX_QUERY_LENGTH = 500