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/guardian/review_session.rs

@@ -17,6 +17,7 @@ use codex_protocol::protocol::EventMsg;
 use codex_protocol::protocol::InitialHistory;
 use codex_protocol::protocol::Op;
 use codex_protocol::protocol::RolloutItem;
+use codex_protocol::protocol::SandboxPolicy;
 use codex_protocol::protocol::SubAgentSource;
 use codex_protocol::user_input::UserInput;
 use serde_json::Value;
@@ -35,7 +36,6 @@ use crate::config::NetworkProxySpec;
 use crate::config::Permissions;
 use crate::config::types::McpServerConfig;
 use crate::model_provider_info::ModelProviderInfo;
-use crate::protocol::SandboxPolicy;
 use crate::rollout::recorder::RolloutRecorder;
 use codex_features::Feature;
 

codex-rs/core/src/guardian/tests.rs

@@ -15,7 +15,6 @@ use crate::config_loader::NetworkDomainPermissionToml;
 use crate::config_loader::NetworkDomainPermissionsToml;
 use crate::config_loader::RequirementSource;
 use crate::config_loader::Sourced;
-use crate::protocol::SandboxPolicy;
 use crate::test_support;
 use codex_network_proxy::NetworkProxyConfig;
 use codex_protocol::approvals::NetworkApprovalProtocol;
@@ -27,6 +26,7 @@ use codex_protocol::protocol::EventMsg;
 use codex_protocol::protocol::GuardianAssessmentStatus;
 use codex_protocol::protocol::GuardianRiskLevel;
 use codex_protocol::protocol::ReviewDecision;
+use codex_protocol::protocol::SandboxPolicy;
 use core_test_support::PathBufExt;
 use core_test_support::TempDirExt;
 use core_test_support::context_snapshot;