chore: Extract PromptStore out of prompt_library (#25837)

One step closer to removing long pole with assistant/assistant2 builds

Release Notes:

- N/A

crates/assistant2/src/buffer_codegen.rs

@@ -14,7 +14,7 @@ use language_model::{
 };
 use multi_buffer::MultiBufferRow;
 use parking_lot::Mutex;
-use prompt_library::PromptBuilder;
+use prompt_store::PromptBuilder;
 use rope::Rope;
 use smol::future::FutureExt;
 use std::{