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/terminal_inline_assistant.rs

@@ -16,7 +16,7 @@ use language_model::{
     report_assistant_event, LanguageModelRegistry, LanguageModelRequest,
     LanguageModelRequestMessage, Role,
 };
-use prompt_library::PromptBuilder;
+use prompt_store::PromptBuilder;
 use std::sync::Arc;
 use telemetry_events::{AssistantEvent, AssistantKind, AssistantPhase};
 use terminal_view::TerminalView;