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_compressor_fallback_update.py

@@ -25,6 +25,8 @@ def _make_agent_with_compressor() -> AIAgent:
         "provider": "openai",
         "model": "gpt-4o",
     }
+    agent._fallback_chain = [agent._fallback_model]
+    agent._fallback_index = 0
 
     # Context compressor with primary model values
     compressor = ContextCompressor(