fix(fallback): skip chain entries matching current provider/model/base_url (#22780)

_try_activate_fallback() walked the chain by index without comparing
the candidate entry against the currently-failing backend. So a
misconfigured chain that listed the same provider+model as the primary,
or two custom_providers entries pointing at the same shim URL, would
loop the same failure 3x for the same backend.

After the fix, advance() skips:
  - entries where (provider, model) match the current agent's
  - entries with a base_url + model matching the current backend
    (catches two custom_providers names pointing at the same shim)

Recursing through self._try_activate_fallback() continues to the next
chain entry; if everything matches, returns False and the caller
moves on without retrying the same broken path.

3 regression tests covering same-provider-same-model skip, same-base_url-
same-model skip, and the all-self-matching-returns-False exhaustion path.

Closes #22548 (the Hermes-side portion). The 120s timeout itself in
the downstream claude-cli shim is a deployment concern documented in
that issue's wherewolf87 comment.

run_agent.py

@@ -8042,6 +8042,32 @@ class AIAgent:
         if not fb_provider or not fb_model:
             return self._try_activate_fallback()  # skip invalid, try next
 
+        # Skip entries that resolve to the current (provider, model) — falling
+        # back to the same backend that just failed loops the failure. Compare
+        # base_url too so two distinct custom_providers entries pointing at the
+        # same shim/proxy URL also dedup. See issue #22548.
+        current_provider = (getattr(self, "provider", "") or "").strip().lower()
+        current_model = (getattr(self, "model", "") or "").strip()
+        current_base_url = str(getattr(self, "base_url", "") or "").rstrip("/").lower()
+        fb_base_url_for_dedup = (fb.get("base_url") or "").strip().rstrip("/").lower()
+        if fb_provider == current_provider and fb_model == current_model:
+            logging.warning(
+                "Fallback skip: chain entry %s/%s matches current provider/model",
+                fb_provider, fb_model,
+            )
+            return self._try_activate_fallback()
+        if (
+            fb_base_url_for_dedup
+            and current_base_url
+            and fb_base_url_for_dedup == current_base_url
+            and fb_model == current_model
+        ):
+            logging.warning(
+                "Fallback skip: chain entry base_url %s matches current backend",
+                fb_base_url_for_dedup,
+            )
+            return self._try_activate_fallback()
+
         # Use centralized router for client construction.
         # raw_codex=True because the main agent needs direct responses.stream()
         # access for Codex providers.