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/core/tests/common/test_codex.rs

@@ -430,6 +430,7 @@ impl TestCodexBuilder {
                 SessionSource::Exec,
                 Arc::clone(&environment_manager),
                 /*analytics_events_client*/ None,
+                thread_store_from_config(&config),
             )
         } else {
             codex_core::test_support::thread_manager_with_models_provider_and_home(
@@ -449,7 +450,6 @@ impl TestCodexBuilder {
                     codex_core::test_support::resume_thread_from_rollout_with_user_shell_override(
                         thread_manager.as_ref(),
                         config.clone(),
-                        thread_store_from_config(&config),
                         path,
                         auth_manager,
                         user_shell_override,
@@ -461,7 +461,6 @@ impl TestCodexBuilder {
                 let auth_manager = codex_core::test_support::auth_manager_from_auth(auth);
                 Box::pin(thread_manager.resume_thread_from_rollout(
                     config.clone(),
-                    thread_store_from_config(&config),
                     path,
                     auth_manager,
                     /*parent_trace*/ None,
@@ -473,18 +472,12 @@ impl TestCodexBuilder {
                     codex_core::test_support::start_thread_with_user_shell_override(
                         thread_manager.as_ref(),
                         config.clone(),
-                        thread_store_from_config(&config),
                         user_shell_override,
                     ),
                 )
                 .await?
             }
-            (None, None) => {
-                Box::pin(
-                    thread_manager.start_thread(config.clone(), thread_store_from_config(&config)),
-                )
-                .await?
-            }
+            (None, None) => Box::pin(thread_manager.start_thread(config.clone())).await?,
         };
 
         Ok(TestCodex {