Make thread store process-scoped (#19474)

- Build one app-server process ThreadStore from startup config and share
it with ThreadManager and CodexMessageProcessor.
- Remove per-thread/fork store reconstruction so effective thread config
cannot switch the persistence backend.
- Add params to ThreadStore create/resume for specifying thread
metadata, since otherwise the metadata from store creation would be used
(incorrectly).

codex-rs/mcp-server/src/message_processor.rs

@@ -4,6 +4,7 @@ use std::sync::Arc;
 use codex_arg0::Arg0DispatchPaths;
 use codex_core::ThreadManager;
 use codex_core::config::Config;
+use codex_core::thread_store_from_config;
 use codex_exec_server::EnvironmentManager;
 use codex_login::AuthManager;
 use codex_login::default_client::USER_AGENT_SUFFIX;
@@ -65,6 +66,7 @@ impl MessageProcessor {
             SessionSource::Mcp,
             environment_manager,
             /*analytics_events_client*/ None,
+            thread_store_from_config(config.as_ref()),
         ));
         Self {
             outgoing,