feat: include NetworkConfig through ExecParams (#11105)

This PR adds the following field to `Config`:

```rust
pub network: Option<NetworkProxy>,
```

Though for the moment, it will always be initialized as `None` (this
will be addressed in a subsequent PR).

This PR does the work to thread `network` through to `execute_exec_env()`, `process_exec_tool_call()`, and `UnifiedExecRuntime.run()` to ensure it is available whenever we span a process.

codex-rs/app-server/src/codex_message_processor.rs

@@ -1636,6 +1636,7 @@ impl CodexMessageProcessor {
             cwd,
             expiration: timeout_ms.into(),
             env,
+            network: self.config.network.clone(),
             sandbox_permissions: SandboxPermissions::UseDefault,
             windows_sandbox_level,
             justification: None,