fix(gateway): reload provider routing and bust cached agents on routing changes

[soynchux]

Summary

Reload provider_routing at agent construction time so live config edits take effect on the next turn, and include routing fields in the gateway agent cache signature so stale agents are not reused after routing changes.

Why

The gateway was holding on to the startup routing snapshot, which meant config edits could leave subsequent turns using an outdated OpenRouter routing policy.

Testing

• 62 passed
• test_provider_routing_change_busts_cache
• test_reads_provider_routing_subkeys
• test_run_agent_reloads_provider_routing_and_busts_cached_agent
• full test_agent_cache.py
• full test_session_model_override_routing.py

[hclsys]

This is the right shape and it's complete — verified both halves:

1. Reload: _refresh_provider_routing() re-reads provider_routing from config and updates self._provider_routing, called at the agent-construction/turn path (run.py:~11596, ~16393) instead of using the startup snapshot. ✓
2. Cache-bust (the part that's easy to get wrong): you added provider_routing.* to _extract_cache_busting_config, and I confirmed that actually feeds the cache key — _agent_config_signature(..., cache_keys=self._extract_cache_busting_config(user_config)) (run.py:16489-16494), and the cache stores (AIAgent, config_signature_str) (1742). So a routing change changes the signature → the cached agent is invalidated and rebuilt, rather than a stale agent being reused. The test_provider_routing_change_busts_cache (sig_before != sig_after) + the first_cached_agent is not second_cached_agent integration assertion prove both the signature change and the actual rebuild.

Worth flagging for the maintainer: this is exactly the pattern the still-open #32007 (reload agent.system_prompt) needs and is currently missing — #32007 reloads the prompt but does not add it to the cache signature, so it only fixes the cache-miss path and leaves returning sessions stale (I flagged that on #32007). If both land, the system-prompt one should mirror this PR's _extract_cache_busting_config approach. This PR itself is correct and well-tested. LGTM.

[teknium1]

Thanks for addressing both the constructor snapshot and cached-agent path; current main still has the reported gap: gateway/run.py:13383 and gateway/run.py:18032 read the startup self._provider_routing, while gateway/run.py:18158-18163 builds the cache signature without routing fields.

Problems

• The new refresh helper would retain a configuration-source mismatch. gateway/run.py:4982-4993 loads raw YAML, but the gateway’s effective config path applies the managed-scope overlay at gateway/run.py:2352-2359. The PR should not construct an agent from raw routing while calculating its cache signature from effective user_config.

Suggested changes

• Thread effective user_config["provider_routing"] through both construction paths, validate it as a dict, and use that same value for cache invalidation. Add coverage for the effective/managed configuration path.

Automated hermes-sweeper review.

[teknium1]

Use the effective user_config already loaded above for this refresh. _load_provider_routing() reads raw YAML, while _load_gateway_config() applies managed scope; otherwise the new routing passed to AIAgent can disagree with the routing values used in the cache signature.