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>

cli.py

@@ -1182,9 +1182,13 @@ class HermesCLI:
         self._provider_require_params = pr.get("require_parameters", False)
         self._provider_data_collection = pr.get("data_collection")
         
-        # Fallback model config — tried when primary provider fails after retries
-        fb = CLI_CONFIG.get("fallback_model") or {}
-        self._fallback_model = fb if fb.get("provider") and fb.get("model") else None
+        # Fallback provider chain — tried in order when primary fails after retries.
+        # Supports new list format (fallback_providers) and legacy single-dict (fallback_model).
+        fb = CLI_CONFIG.get("fallback_providers") or CLI_CONFIG.get("fallback_model") or []
+        # Normalize legacy single-dict to a one-element list
+        if isinstance(fb, dict):
+            fb = [fb] if fb.get("provider") and fb.get("model") else []
+        self._fallback_model = fb
 
         # Optional cheap-vs-strong routing for simple turns
         self._smart_model_routing = CLI_CONFIG.get("smart_model_routing", {}) or {}