fix: clear api_mode on provider switch instead of hardcoding chat_completions (#3857)

PR #3726 fixed stale codex_responses persisting when switching providers
by hardcoding api_mode=chat_completions in 5 model flows. This broke
MiniMax, MiniMax-CN, and Alibaba which use /anthropic endpoints that
need anthropic_messages — the hardcoded value overrides the URL-based
auto-detection in runtime_provider.py.

Fix: pop api_mode from config in the 3 URL-dependent flows (custom
endpoint, Kimi, api_key_provider) instead of hardcoding. The runtime
resolver already correctly auto-detects api_mode from the base_url
suffix (/anthropic -> anthropic_messages, else chat_completions).

OpenRouter and Copilot ACP flows keep the explicit value since their
api_mode is always known.

Reported by stefan171.

hermes_cli/main.py

@@ -1269,7 +1269,7 @@ def _model_flow_custom(config):
             cfg["model"] = model
         model["provider"] = "custom"
         model["base_url"] = effective_url
-        model["api_mode"] = "chat_completions"
+        model.pop("api_mode", None)  # let runtime auto-detect from URL
         save_config(cfg)
         deactivate_provider()
 
@@ -2051,7 +2051,7 @@ def _model_flow_kimi(config, current_model=""):
             cfg["model"] = model
         model["provider"] = provider_id
         model["base_url"] = effective_base
-        model["api_mode"] = "chat_completions"
+        model.pop("api_mode", None)  # let runtime auto-detect from URL
         save_config(cfg)
         deactivate_provider()
 
@@ -2158,7 +2158,7 @@ def _model_flow_api_key_provider(config, provider_id, current_model=""):
             cfg["model"] = model
         model["provider"] = provider_id
         model["base_url"] = effective_base
-        model["api_mode"] = "chat_completions"
+        model.pop("api_mode", None)  # let runtime auto-detect from URL
         save_config(cfg)
         deactivate_provider()