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/linux-sandbox/tests/suite/landlock.rs

@@ -75,6 +75,7 @@ async fn run_cmd_result_with_writable_roots(
         cwd,
         expiration: timeout_ms.into(),
         env: create_env_from_core_vars(),
+        network: None,
         sandbox_permissions: SandboxPermissions::UseDefault,
         windows_sandbox_level: WindowsSandboxLevel::Disabled,
         justification: None,
@@ -231,6 +232,7 @@ async fn assert_network_blocked(cmd: &[&str]) {
         // do not stall the suite.
         expiration: NETWORK_TIMEOUT_MS.into(),
         env: create_env_from_core_vars(),
+        network: None,
         sandbox_permissions: SandboxPermissions::UseDefault,
         windows_sandbox_level: WindowsSandboxLevel::Disabled,
         justification: None,