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/src/codex.rs

@@ -6658,6 +6658,7 @@ mod tests {
             cwd: turn_context.cwd.clone(),
             expiration: timeout_ms.into(),
             env: HashMap::new(),
+            network: None,
             sandbox_permissions,
             windows_sandbox_level: turn_context.windows_sandbox_level,
             justification: Some("test".to_string()),
@@ -6670,6 +6671,7 @@ mod tests {
             cwd: params.cwd.clone(),
             expiration: timeout_ms.into(),
             env: HashMap::new(),
+            network: None,
             windows_sandbox_level: turn_context.windows_sandbox_level,
             justification: params.justification.clone(),
             arg0: None,