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

[teknium1]

Summary

Salvage of #1761 by @uzaylisak. Extends the single fallback_model mechanism into an ordered provider chain. When the primary model fails (rate limit, 4xx, connection error), Hermes tries each fallback in sequence until one succeeds.

Closes #1734.

Config

# New list format — tried in order
fallback_providers:
  - provider: openrouter
    model: anthropic/claude-sonnet-4
  - provider: openai
    model: gpt-4o
  - provider: zai
    model: glm-4.7

Legacy single-dict fallback_model format still works unchanged.

Key fix vs original PR

The original PR modified _try_activate_fallback() to use a chain index but did NOT update the call sites. Several call sites guarded with not self._fallback_activated, which prevented the chain from ever advancing past provider #1. This salvage replaces those guards with self._fallback_index < len(self._fallback_chain) so the chain actually works.

Additionally, when a provider in the chain fails to resolve (unconfigured, auth error, etc.), the chain now skips to the next entry instead of stopping.

Changes

File	Change
run_agent.py	_fallback_chain + _fallback_index replaces one-shot _fallback_model; call sites updated
cli.py	Reads fallback_providers with legacy fallback_model compat
gateway/run.py	Same
hermes_cli/config.py	fallback_providers: [] in DEFAULT_CONFIG
tests/test_provider_fallback.py	12 new tests for chain init, advancement, skip behavior
tests/test_run_agent.py	5 existing test fixtures updated for new attributes
tests/test_compressor_fallback_update.py	1 fixture updated

Live test results

Tested with real OpenRouter API — primary model 404s, chain advances through fallbacks:

Primary: openai/fake-model-1 → 400 "not a valid model ID"
  ⚠️ Non-retryable error (HTTP 400) — trying fallback...
  🔄 switching to: openai/fake-model-2 (openrouter)
Fallback #1: openai/fake-model-2 → 400 "not a valid model ID"
  ⚠️ Non-retryable error (HTTP 400) — trying fallback...
  🔄 switching to: anthropic/claude-sonnet-4 (openrouter)
Fallback #2: anthropic/claude-sonnet-4 → ✓ "chain works"

Test results

6806 passed, 9 pre-existing failures, 0 regressions.