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/exec-server/src/posix/escalate_server.rs

@@ -87,6 +87,7 @@ impl EscalateServer {
                 cwd: PathBuf::from(&workdir),
                 expiration: ExecExpiration::Cancellation(cancel_rx),
                 env,
+                network: None,
                 sandbox_permissions: SandboxPermissions::UseDefault,
                 windows_sandbox_level: WindowsSandboxLevel::Disabled,
                 justification: None,