feat(providers): add ordered fallback provider chain (salvage #1761) (#3813)

Extends the single fallback_model mechanism into an ordered chain.
When the primary model fails, Hermes tries each fallback provider in
sequence until one succeeds or the chain is exhausted.

Config format (new):
  fallback_providers:
    - provider: openrouter
      model: anthropic/claude-sonnet-4
    - provider: openai
      model: gpt-4o

Legacy single-dict fallback_model format still works unchanged.

Key fix vs original PR: the call sites in the retry loop now use
_fallback_index < len(_fallback_chain) instead of the old one-shot
_fallback_activated guard, so the chain actually advances through
all configured providers.

Changes:
- run_agent.py: _fallback_chain list + _fallback_index replaces
  one-shot _fallback_model; _try_activate_fallback() advances
  through chain; failed provider resolution skips to next entry;
  call sites updated to allow chain advancement
- cli.py: reads fallback_providers with legacy fallback_model compat
- gateway/run.py: same
- hermes_cli/config.py: fallback_providers: [] in DEFAULT_CONFIG
- tests: 12 new chain tests + 6 existing test fixtures updated

Co-authored-by: uzaylisak <uzaylisak@users.noreply.github.com>

gateway/run.py

@@ -919,11 +919,12 @@ class GatewayRunner:
         return {}
 
     @staticmethod
-    def _load_fallback_model() -> dict | None:
-        """Load fallback model config from config.yaml.
+    def _load_fallback_model() -> list | dict | None:
+        """Load fallback provider chain from config.yaml.
 
-        Returns a dict with 'provider' and 'model' keys, or None if
-        not configured / both fields empty.
+        Returns a list of provider dicts (``fallback_providers``), a single
+        dict (legacy ``fallback_model``), or None if not configured.
+        AIAgent.__init__ normalizes both formats into a chain.
         """
         try:
             import yaml as _y
@@ -931,8 +932,8 @@ class GatewayRunner:
             if cfg_path.exists():
                 with open(cfg_path, encoding="utf-8") as _f:
                     cfg = _y.safe_load(_f) or {}
-                fb = cfg.get("fallback_model", {}) or {}
-                if fb.get("provider") and fb.get("model"):
+                fb = cfg.get("fallback_providers") or cfg.get("fallback_model") or None
+                if fb:
                     return fb
         except Exception:
             pass