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/core/tests/suite/exec.rs

@@ -36,6 +36,7 @@ async fn run_test_cmd(tmp: TempDir, cmd: Vec<&str>) -> Result<ExecToolCallOutput
         cwd: tmp.path().to_path_buf(),
         expiration: 1000.into(),
         env: HashMap::new(),
+        network: None,
         sandbox_permissions: SandboxPermissions::UseDefault,
         windows_sandbox_level: WindowsSandboxLevel::Disabled,
         justification: None,