fix(/model): accept provider switches when /models is unreachable

Gateway /model <name> --provider opencode-go (or any provider whose /models
endpoint is down, 404s, or doesn't exist) silently failed. validate_requested_model
returned accepted=False whenever fetch_api_models returned None, switch_model
returned success=False, and the gateway never wrote _session_model_overrides —
so the switch appeared to succeed in the error message flow but the next turn
kept calling the old provider.

The validator already had static-catalog fallbacks for MiniMax and Codex
(providers without a /models endpoint). Extended the same pattern as the
terminal fallback: when the live probe fails, consult provider_model_ids()
for the curated catalog. Known models → accepted+recognized. Close typos →
auto-corrected. Unknown models → soft-accepted with a 'Not in curated
catalog' warning. Providers with no catalog at all → soft-accepted with a
generic 'Note:' warning, finally honoring the in-code comment ('Accept and
persist, but warn') that had been lying since it was written.

Tests: 7 new tests in test_opencode_go_validation_fallback.py covering the
catalog lookup, case-insensitive match, auto-correct, unknown-with-suggestion,
unknown-without-suggestion, and no-catalog paths. TestValidateApiFallback in
test_model_validation.py updated — its four 'rejected_when_api_down' tests
were encoding exactly the bug being fixed.

hermes_cli/models.py

@@ -2426,13 +2426,70 @@ def validate_requested_model(
         except Exception:
             pass  # Fall through to generic warning
 
+    # Static-catalog fallback: when the /models probe was unreachable,
+    # validate against the curated list from provider_model_ids() — same
+    # pattern as the openai-codex and minimax branches above.  This fixes
+    # /model switches in the gateway for providers like opencode-go and
+    # opencode-zen whose /models endpoint returns 404 against the HTML
+    # marketing site.  Without this block, validate_requested_model would
+    # reject every model on such providers, switch_model() would return
+    # success=False, and the gateway would never write to
+    # _session_model_overrides.
     provider_label = _PROVIDER_LABELS.get(normalized, normalized)
+    try:
+        catalog_models = provider_model_ids(normalized)
+    except Exception:
+        catalog_models = []
+
+    if catalog_models:
+        catalog_lower = {m.lower(): m for m in catalog_models}
+        if requested_for_lookup.lower() in catalog_lower:
+            return {
+                "accepted": True,
+                "persist": True,
+                "recognized": True,
+                "message": None,
+            }
+        catalog_lower_list = list(catalog_lower.keys())
+        auto = get_close_matches(
+            requested_for_lookup.lower(), catalog_lower_list, n=1, cutoff=0.9
+        )
+        if auto:
+            corrected = catalog_lower[auto[0]]
+            return {
+                "accepted": True,
+                "persist": True,
+                "recognized": True,
+                "corrected_model": corrected,
+                "message": f"Auto-corrected `{requested}` → `{corrected}`",
+            }
+        suggestions = get_close_matches(
+            requested_for_lookup.lower(), catalog_lower_list, n=3, cutoff=0.5
+        )
+        suggestion_text = ""
+        if suggestions:
+            suggestion_text = "\n  Similar models: " + ", ".join(
+                f"`{catalog_lower[s]}`" for s in suggestions
+            )
+        return {
+            "accepted": True,
+            "persist": True,
+            "recognized": False,
+            "message": (
+                f"Note: `{requested}` was not found in the {provider_label} curated catalog "
+                f"and the /models endpoint was unreachable.{suggestion_text}"
+                f"\n  The model may still work if it exists on the provider."
+            ),
+        }
+
+    # No catalog available — accept with a warning, matching the comment's
+    # stated intent ("Accept and persist, but warn").
     return {
-        "accepted": False,
-        "persist": False,
+        "accepted": True,
+        "persist": True,
         "recognized": False,
         "message": (
-            f"Could not reach the {provider_label} API to validate `{requested}`. "
+            f"Note: could not reach the {provider_label} API to validate `{requested}`. "
             f"If the service isn't down, this model may not be valid."
         ),
     }