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/tools/runtimes/shell/unix_escalation_tests.rs

@@ -6,10 +6,6 @@ use super::evaluate_intercepted_exec_policy;
 use super::extract_shell_script;
 use super::join_program_and_argv;
 use super::map_exec_result;
-use crate::protocol::AskForApproval;
-use crate::protocol::GranularApprovalConfig;
-use crate::protocol::ReadOnlyAccess;
-use crate::protocol::SandboxPolicy;
 use crate::sandboxing::SandboxPermissions;
 use codex_execpolicy::Decision;
 use codex_execpolicy::Evaluation;
@@ -23,6 +19,10 @@ use codex_protocol::permissions::FileSystemSandboxEntry;
 use codex_protocol::permissions::FileSystemSandboxPolicy;
 use codex_protocol::permissions::FileSystemSpecialPath;
 use codex_protocol::permissions::NetworkSandboxPolicy;
+use codex_protocol::protocol::AskForApproval;
+use codex_protocol::protocol::GranularApprovalConfig;
+use codex_protocol::protocol::ReadOnlyAccess;
+use codex_protocol::protocol::SandboxPolicy;
 use codex_sandboxing::SandboxType;
 use codex_shell_escalation::EscalationExecution;
 use codex_shell_escalation::EscalationPermissions;