feat: identity-aware rejected-token marker for LLM OAuth cache

distrust_access_token compare-and-invalidates the in-memory entry only
when the cached token equals the rejected one, so a concurrent refresh
is never clobbered. is_valid_access_token and both expiry checks in
prepare_oauth_access_token treat marked tokens as expired, forcing a
refresh of provider-rejected tokens that are still locally unexpired.
The marker is cleared after every completed refresh, including ones
that return the same token.

src/client/access_token.rs

@@ -2,6 +2,7 @@ use anyhow::{Result, anyhow};
 use chrono::Utc;
 use indexmap::IndexMap;
 use parking_lot::RwLock;
+use std::collections::HashMap;
 use std::sync::LazyLock;
 
 type AccessTokenEntry = (String, i64, Option<String>);
@@ -9,6 +10,12 @@ type AccessTokenEntry = (String, i64, Option<String>);
 static ACCESS_TOKENS: LazyLock<RwLock<IndexMap<String, AccessTokenEntry>>> =
     LazyLock::new(|| RwLock::new(IndexMap::new()));
 
+/// Tokens a provider rejected (401) despite being locally unexpired.
+/// Maps client name → the exact rejected token so a concurrently-refreshed
+/// different token is never distrusted by mistake.
+static REJECTED_TOKENS: LazyLock<RwLock<HashMap<String, String>>> =
+    LazyLock::new(|| RwLock::new(HashMap::new()));
+
 pub fn get_access_token(client_name: &str) -> Result<String> {
     ACCESS_TOKENS
         .read()
@@ -30,7 +37,7 @@ pub fn is_valid_access_token(client_name: &str) -> bool {
         Some(v) => v,
         None => return false,
     };
-    !token.is_empty() && Utc::now().timestamp() < *expires_at
+    !token.is_empty() && Utc::now().timestamp() < *expires_at && !is_rejected(client_name, token)
 }
 
 pub fn set_access_token(
@@ -45,3 +52,112 @@ pub fn set_access_token(
     entry.1 = expires_at;
     entry.2 = account_id;
 }
+
+/// Compare-and-invalidate a provider-rejected token.
+///
+/// Only if the currently-cached token EQUALS `rejected` is the cache entry
+/// removed and the rejection marker recorded; a concurrently-refreshed
+/// different token is left untouched and no marker is set.
+///
+/// Returns true if a cache entry existed for this client at all (whether or
+/// not it matched `rejected`) — i.e. the client is token-authed and a retry
+/// after refresh is worthwhile. Returns false when there is no entry
+/// (API-key clients).
+#[allow(dead_code)] // Called by the 401-retry path once it lands.
+pub fn distrust_access_token(client_name: &str, rejected: &str) -> bool {
+    let mut access_tokens = ACCESS_TOKENS.write();
+    let (token, _, _) = match access_tokens.get(client_name) {
+        Some(v) => v,
+        None => return false,
+    };
+    if token == rejected {
+        access_tokens.shift_remove(client_name);
+        REJECTED_TOKENS
+            .write()
+            .insert(client_name.to_string(), rejected.to_string());
+    }
+    true
+}
+
+pub fn is_rejected(client_name: &str, token: &str) -> bool {
+    REJECTED_TOKENS
+        .read()
+        .get(client_name)
+        .is_some_and(|rejected| rejected == token)
+}
+
+pub fn clear_rejected(client_name: &str) {
+    REJECTED_TOKENS.write().remove(client_name);
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn distrust_removes_matching_token_and_sets_marker() {
+        let client = "distrust-match-test";
+        set_access_token(client, "at-1".into(), Utc::now().timestamp() + 3600, None);
+
+        assert!(distrust_access_token(client, "at-1"));
+
+        assert!(get_access_token(client).is_err(), "cache entry not removed");
+        assert!(is_rejected(client, "at-1"), "marker not set");
+    }
+
+    #[test]
+    fn distrust_keeps_differing_token_and_skips_marker() {
+        let client = "distrust-differ-test";
+        set_access_token(client, "at-new".into(), Utc::now().timestamp() + 3600, None);
+
+        assert!(distrust_access_token(client, "at-old"));
+
+        assert_eq!(get_access_token(client).unwrap(), "at-new");
+        assert!(!is_rejected(client, "at-old"), "marker set for stale token");
+    }
+
+    #[test]
+    fn distrust_returns_false_without_cache_entry() {
+        let client = "distrust-missing-test";
+
+        assert!(!distrust_access_token(client, "at-1"));
+        assert!(!is_rejected(client, "at-1"));
+    }
+
+    #[test]
+    fn is_valid_access_token_false_for_rejected_token() {
+        let client = "rejected-valid-test";
+        set_access_token(client, "at-1".into(), Utc::now().timestamp() + 3600, None);
+        assert!(is_valid_access_token(client));
+
+        distrust_access_token(client, "at-1");
+        // A concurrent in-flight prepare re-caches the rejected file token
+        // between mark and refresh; it must still be treated as invalid.
+        set_access_token(client, "at-1".into(), Utc::now().timestamp() + 3600, None);
+
+        assert!(!is_valid_access_token(client));
+    }
+
+    #[test]
+    fn clear_rejected_clears_marker_and_clients_are_isolated() {
+        let client_a = "rejected-isolation-a";
+        let client_b = "rejected-isolation-b";
+        set_access_token(client_a, "at-1".into(), Utc::now().timestamp() + 3600, None);
+        distrust_access_token(client_a, "at-1");
+
+        assert!(is_rejected(client_a, "at-1"));
+        assert!(
+            !is_rejected(client_b, "at-1"),
+            "marker leaked across clients"
+        );
+
+        clear_rejected(client_b);
+        assert!(
+            is_rejected(client_a, "at-1"),
+            "wrong client's marker cleared"
+        );
+
+        clear_rejected(client_a);
+        assert!(!is_rejected(client_a, "at-1"));
+    }
+}