fix(copilot): pass two-slash custom-model ids through unchanged

Copilot enterprise custom models (BYOK) expose catalog ids shaped
owner/sub/model. normalize_copilot_model_id() only tried the full id and the
id minus its FIRST segment, and returned the stripped form when neither
matched the (unreachable) catalog - corrupting those ids into sub/model,
which the CLI's second normalization pass then stripped again to model. The
Copilot API answered HTTP 400 model_not_supported on every call.

Only accept the strip guess when the remainder is itself a flat id: a result
that still contains "/" cannot be a Copilot id.

Fixes #110597

hermes_cli/models.py

@@ -2051,7 +2051,12 @@ def normalize_copilot_model_id(
             return candidate
 
     if "/" in raw:
-        return raw.split("/", 1)[1].strip()
+        stripped = raw.split("/", 1)[1].strip()
+        # Enterprise BYOK custom models expose ``owner/sub/model`` ids (two
+        # slashes). A strip guess that still contains "/" cannot be a Copilot
+        # id, so pass the input through untouched instead of corrupting it.
+        if stripped and "/" not in stripped:
+            return stripped
     return raw