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>

tests/test_run_agent.py

@@ -2507,6 +2507,8 @@ class TestFallbackAnthropicProvider:
     def test_fallback_to_anthropic_sets_api_mode(self, agent):
         agent._fallback_activated = False
         agent._fallback_model = {"provider": "anthropic", "model": "claude-sonnet-4-20250514"}
+        agent._fallback_chain = [agent._fallback_model]
+        agent._fallback_index = 0
 
         mock_client = MagicMock()
         mock_client.base_url = "https://api.anthropic.com/v1"
@@ -2528,6 +2530,8 @@ class TestFallbackAnthropicProvider:
     def test_fallback_to_anthropic_enables_prompt_caching(self, agent):
         agent._fallback_activated = False
         agent._fallback_model = {"provider": "anthropic", "model": "claude-sonnet-4-20250514"}
+        agent._fallback_chain = [agent._fallback_model]
+        agent._fallback_index = 0
 
         mock_client = MagicMock()
         mock_client.base_url = "https://api.anthropic.com/v1"
@@ -2545,6 +2549,8 @@ class TestFallbackAnthropicProvider:
     def test_fallback_to_openrouter_uses_openai_client(self, agent):
         agent._fallback_activated = False
         agent._fallback_model = {"provider": "openrouter", "model": "anthropic/claude-sonnet-4"}
+        agent._fallback_chain = [agent._fallback_model]
+        agent._fallback_index = 0
 
         mock_client = MagicMock()
         mock_client.base_url = "https://openrouter.ai/api/v1"
@@ -3238,6 +3244,8 @@ class TestFallbackSetsOAuthFlag:
     def test_fallback_to_anthropic_oauth_sets_flag(self, agent):
         agent._fallback_activated = False
         agent._fallback_model = {"provider": "anthropic", "model": "claude-sonnet-4-6"}
+        agent._fallback_chain = [agent._fallback_model]
+        agent._fallback_index = 0
 
         mock_client = MagicMock()
         mock_client.base_url = "https://api.anthropic.com/v1"
@@ -3259,6 +3267,8 @@ class TestFallbackSetsOAuthFlag:
     def test_fallback_to_anthropic_api_key_clears_flag(self, agent):
         agent._fallback_activated = False
         agent._fallback_model = {"provider": "anthropic", "model": "claude-sonnet-4-6"}
+        agent._fallback_chain = [agent._fallback_model]
+        agent._fallback_index = 0
 
         mock_client = MagicMock()
         mock_client.base_url = "https://api.anthropic.com/v1"