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/config/mod.rs

@@ -1,4 +1,3 @@
-use crate::auth::AuthCredentialsStoreMode;
 use crate::config::edit::ConfigEdit;
 use crate::config::edit::ConfigEditsBuilder;
 use crate::config::types::AppsConfigToml;
@@ -50,9 +49,6 @@ use crate::model_provider_info::built_in_model_providers;
 use crate::path_utils::normalize_for_native_workdir;
 use crate::project_doc::DEFAULT_PROJECT_DOC_FILENAME;
 use crate::project_doc::LOCAL_PROJECT_DOC_FILENAME;
-use crate::protocol::AskForApproval;
-use crate::protocol::ReadOnlyAccess;
-use crate::protocol::SandboxPolicy;
 use crate::unified_exec::DEFAULT_MAX_BACKGROUND_TERMINAL_TIMEOUT_MS;
 use crate::unified_exec::MIN_EMPTY_YIELD_TIME_MS;
 use crate::windows_sandbox::WindowsSandboxLevelExt;
@@ -66,6 +62,7 @@ use codex_features::FeatureOverrides;
 use codex_features::Features;
 use codex_features::FeaturesToml;
 use codex_git_utils::resolve_root_git_project_for_trust;
+use codex_login::AuthCredentialsStoreMode;
 use codex_mcp::mcp::McpConfig;
 use codex_protocol::config_types::AltScreenMode;
 use codex_protocol::config_types::ForcedLoginMethod;
@@ -83,6 +80,9 @@ use codex_protocol::openai_models::ModelsResponse;
 use codex_protocol::openai_models::ReasoningEffort;
 use codex_protocol::permissions::FileSystemSandboxPolicy;
 use codex_protocol::permissions::NetworkSandboxPolicy;
+use codex_protocol::protocol::AskForApproval;
+use codex_protocol::protocol::ReadOnlyAccess;
+use codex_protocol::protocol::SandboxPolicy;
 use codex_rmcp_client::OAuthCredentialsStoreMode;
 use codex_utils_absolute_path::AbsolutePathBuf;
 use codex_utils_absolute_path::AbsolutePathBufGuard;

codex-rs/core/src/config/profile.rs

@@ -7,7 +7,6 @@ use crate::config::ToolsToml;
 use crate::config::types::ApprovalsReviewer;
 use crate::config::types::Personality;
 use crate::config::types::WindowsToml;
-use crate::protocol::AskForApproval;
 use codex_features::FeaturesToml;
 use codex_protocol::config_types::ReasoningSummary;
 use codex_protocol::config_types::SandboxMode;
@@ -15,6 +14,7 @@ use codex_protocol::config_types::ServiceTier;
 use codex_protocol::config_types::Verbosity;
 use codex_protocol::config_types::WebSearchMode;
 use codex_protocol::openai_models::ReasoningEffort;
+use codex_protocol::protocol::AskForApproval;
 
 /// Collection of common configuration options that a user can define as a unit
 /// in `config.toml`.