core: remove cross-crate re-exports from lib.rs (#16512)

## Why

`codex-core` was re-exporting APIs owned by sibling `codex-*` crates,
which made downstream crates depend on `codex-core` as a proxy module
instead of the actual owner crate.

Removing those forwards makes crate boundaries explicit and lets leaf
crates drop unnecessary `codex-core` dependencies. In this PR, this
reduces the dependency on `codex-core` to `codex-login` in the following
files:

```
codex-rs/backend-client/Cargo.toml
codex-rs/mcp-server/tests/common/Cargo.toml
```

## What

- Remove `codex-rs/core/src/lib.rs` re-exports for symbols owned by
`codex-login`, `codex-mcp`, `codex-rollout`, `codex-analytics`,
`codex-protocol`, `codex-shell-command`, `codex-sandboxing`,
`codex-tools`, and `codex-utils-path`.
- Delete the `default_client` forwarding shim in `codex-rs/core`.
- Update in-crate and downstream callsites to import directly from the
owning `codex-*` crate.
- Add direct Cargo dependencies where callsites now target the owner
crate, and remove `codex-core` from `codex-rs/backend-client`.

codex-rs/core/src/lib.rs

@@ -9,7 +9,6 @@ pub mod api_bridge;
 mod apply_patch;
 mod apps;
 mod arc_monitor;
-pub use codex_login as auth;
 mod auth_env_telemetry;
 mod client;
 mod client_common;
@@ -30,7 +29,6 @@ pub mod config_loader;
 pub mod connectors;
 mod context_manager;
 mod contextual_user_message;
-pub use codex_utils_path::env;
 mod environment_context;
 pub mod error;
 pub mod exec;
@@ -52,10 +50,6 @@ pub mod models_manager;
 mod network_policy_decision;
 pub mod network_proxy_loader;
 mod original_image_detail;
-pub use codex_mcp::mcp_connection_manager;
-pub use codex_mcp::mcp_connection_manager::MCP_SANDBOX_STATE_CAPABILITY;
-pub use codex_mcp::mcp_connection_manager::MCP_SANDBOX_STATE_METHOD;
-pub use codex_mcp::mcp_connection_manager::SandboxState;
 pub use text_encoding::bytes_to_string_smart;
 mod mcp_tool_call;
 mod memories;
@@ -105,7 +99,6 @@ mod text_encoding;
 mod unified_exec;
 pub mod windows_sandbox;
 pub use client::X_RESPONSESAPI_INCLUDE_TIMING_METRICS_HEADER;
-pub use codex_protocol::config_types::ModelProviderAuthInfo;
 pub use model_provider_info::DEFAULT_LMSTUDIO_PORT;
 pub use model_provider_info::DEFAULT_OLLAMA_PORT;
 pub use model_provider_info::LMSTUDIO_OSS_PROVIDER_ID;
@@ -131,20 +124,6 @@ pub type ConversationManager = ThreadManager;
 pub type NewConversation = NewThread;
 #[deprecated(note = "use CodexThread")]
 pub type CodexConversation = CodexThread;
-// Re-export common auth types for workspace consumers
-pub use auth::AuthManager;
-pub use auth::CodexAuth;
-pub use codex_analytics::AnalyticsEventsClient;
-pub use codex_analytics::AppServerRpcTransport;
-mod default_client_forwarding;
-
-/// Default Codex HTTP client headers and reqwest construction.
-///
-/// Implemented in [`codex_login::default_client`]; this module re-exports that API for crates
-/// that import `codex_core::default_client`.
-pub mod default_client {
-    pub use super::default_client_forwarding::*;
-}
 pub mod project_doc;
 mod rollout;
 pub(crate) mod safety;
@@ -154,7 +133,6 @@ pub mod shell;
 pub mod shell_snapshot;
 pub mod spawn;
 pub mod state_db_bridge;
-pub use codex_rollout::state_db;
 mod thread_rollout_truncation;
 mod tools;
 pub mod turn_diff_tracker;
@@ -188,12 +166,6 @@ mod state;
 mod tasks;
 mod user_shell_command;
 pub mod util;
-pub(crate) use codex_protocol::protocol;
-pub(crate) use codex_shell_command::bash;
-pub(crate) use codex_shell_command::is_dangerous_command;
-pub(crate) use codex_shell_command::is_safe_command;
-pub(crate) use codex_shell_command::parse_command;
-pub(crate) use codex_shell_command::powershell;
 
 pub use client::ModelClient;
 pub use client::ModelClientSession;
@@ -202,8 +174,6 @@ pub use client_common::Prompt;
 pub use client_common::REVIEW_PROMPT;
 pub use client_common::ResponseEvent;
 pub use client_common::ResponseStream;
-pub use codex_sandboxing::get_platform_sandbox;
-pub use codex_tools::parse_tool_input_schema;
 pub use compact::content_items_to_text;
 pub use event_mapping::parse_turn_item;
 pub use exec_policy::ExecPolicyError;