fix: address CI failures and reviewer feedback on #3214

- Add missing retry_after: None field to ApiError::Api construction
  in main.rs test. This field was introduced by the Retry-After
  header support but was not added to the test's error initializer,
  causing a compile error under CI's strict mode.

- Remove duplicate #[must_use] attribute on retry_after() method
  in error.rs (lines 134+138 both had it; kept the outer one
  above the doc comment per convention).

- Cargo fmt --all run.

- Reviewer question "Are defaults preserved?" — answered yes:
  ApiTimeoutConfig defaults to 30s connect / 300s request / 8 retries.
  with_retry_policy() is opt-in. No behavior change without explicit
  configuration.

rust/crates/rusty-claude-cli/src/main.rs

@@ -11693,6 +11693,7 @@ mod tests {
             body: String::new(),
             retryable: false,
             suggested_action: None,
+            retry_after: None,
         };
 
         let rendered = format_user_visible_api_error("session-issue-32", &error);
@@ -11728,7 +11729,7 @@ mod tests {
                 retryable: false,
                 suggested_action: None,
                 retry_after: None,
-}),
+            }),
         };
 
         let rendered = format_user_visible_api_error("session-issue-32", &error);