refactor: reuse is_auth_error from provider.rs, DRY backend retry logic

crates/zesdex-backend/src/app/subagent/engine.rs

@@ -25,19 +25,12 @@ fn step_retry_delay(attempt: u32) -> Duration {
 
 /// Heuristic to decide whether the error is worth retrying.
 fn should_retry_subagent_step(err_str: &str) -> bool {
-    let lower = err_str.to_lowercase();
     // Never retry auth/billing failures
-    if err_str.contains("API error 401")
-        || err_str.contains("API error 402")
-        || err_str.contains("API error 403")
-        || lower.contains("unauthorized")
-        || lower.contains("forbidden")
-        || lower.contains("authentication failed")
-    {
+    if crate::service::provider::is_auth_error(err_str) {
         return false;
     }
     // Never retry abort or user cancellation
-    if lower.contains("aborted") {
+    if err_str.to_lowercase().contains("aborted") {
         return false;
     }
     // Everything else (timeout, 5xx, rate-limit, network blip) is retryable

crates/zesdex-backend/src/app/workflow/engine/mod.rs

@@ -421,9 +421,7 @@ fn spawn_single_agent(sp: SpawnCtx<'_>) -> anyhow::Result<String> {
                 }
                 Err(e) => {
                     let err_str = e.to_string();
-                    let is_auth = err_str.contains("API error 401")
-                        || err_str.contains("API error 402")
-                        || err_str.contains("API error 403");
+                    let is_auth = crate::service::provider::is_auth_error(&err_str);
                     // Auth errors are permanent — don't retry.
                     if is_auth || attempt >= 2 {
                         let _ = done_tx.send(Err(e));

crates/zesdex-backend/src/service/provider.rs

@@ -57,7 +57,7 @@ fn backoff_duration(attempt: u32) -> Duration {
 /// request builders below, plus well-known auth keywords in case the body
 /// contains them.  This is intentionally tighter than `contains("401")`,
 /// which could false-positive on a URL port, model name, or body text.
-fn is_auth_error(err_str: &str) -> bool {
+pub fn is_auth_error(err_str: &str) -> bool {
     let err_lower = err_str.to_lowercase();
     // Structured HTTP status patterns
     (err_str.contains("API error 401")