Move codex module under session (#18249)

## Summary
- rename the core codex module root to session/mod.rs without using
#[path]
- move the codex module directory and tests under core/src/session
- remove session/mod.rs reexports so call sites use explicit child
module paths

## Testing
- cargo test -p codex-core --lib
- cargo check -p codex-core --tests
- just fmt
- just fix -p codex-core
- git diff --check

codex-rs/core/src/connectors.rs

@@ -21,12 +21,12 @@ use serde::Deserialize;
 use serde::de::DeserializeOwned;
 use tracing::warn;
 
-use crate::codex::INITIAL_SUBMIT_ID;
 use crate::config::Config;
 use crate::config_loader::AppsRequirementsToml;
 use crate::mcp::McpManager;
 use crate::plugins::PluginsManager;
 use crate::plugins::list_tool_suggest_discoverable_plugins;
+use crate::session::INITIAL_SUBMIT_ID;
 use codex_config::types::AppToolApproval;
 use codex_config::types::AppsConfigToml;
 use codex_config::types::ToolSuggestDiscoverableType;