fix(config): restore custom providers after v11→v12 migration

The v11→v12 migration converts custom_providers (list) into providers
(dict), then deletes the list. But all runtime resolvers read from
custom_providers — after migration, named custom endpoints silently stop
resolving and fallback chains fail with AuthError.

Add get_compatible_custom_providers() that reads from both config schemas
(legacy custom_providers list + v12+ providers dict), normalizes entries,
deduplicates, and returns a unified list. Update ALL consumers:

- hermes_cli/runtime_provider.py: _get_named_custom_provider() + key_env
- hermes_cli/auth_commands.py: credential pool provider names
- hermes_cli/main.py: model picker + _model_flow_named_custom()
- agent/auxiliary_client.py: key_env + custom_entry model fallback
- agent/credential_pool.py: _iter_custom_providers()
- cli.py + gateway/run.py: /model switch custom_providers passthrough
- run_agent.py + gateway/run.py: per-model context_length lookup

Also: use config.pop() instead of del for safer migration, fix stale
_config_version assertions in tests, add pool mock to codex test.

Co-authored-by: 墨綠BG <s5460703@gmail.com>
Closes #8776, salvaged from PR #8814

run_agent.py

@@ -1267,24 +1267,29 @@ class AIAgent:
 
         # Check custom_providers per-model context_length
         if _config_context_length is None:
-            _custom_providers = _agent_cfg.get("custom_providers")
-            if isinstance(_custom_providers, list):
-                for _cp_entry in _custom_providers:
-                    if not isinstance(_cp_entry, dict):
-                        continue
-                    _cp_url = (_cp_entry.get("base_url") or "").rstrip("/")
-                    if _cp_url and _cp_url == self.base_url.rstrip("/"):
-                        _cp_models = _cp_entry.get("models", {})
-                        if isinstance(_cp_models, dict):
-                            _cp_model_cfg = _cp_models.get(self.model, {})
-                            if isinstance(_cp_model_cfg, dict):
-                                _cp_ctx = _cp_model_cfg.get("context_length")
-                                if _cp_ctx is not None:
-                                    try:
-                                        _config_context_length = int(_cp_ctx)
-                                    except (TypeError, ValueError):
-                                        pass
-                        break
+            try:
+                from hermes_cli.config import get_compatible_custom_providers
+                _custom_providers = get_compatible_custom_providers(_agent_cfg)
+            except Exception:
+                _custom_providers = _agent_cfg.get("custom_providers")
+                if not isinstance(_custom_providers, list):
+                    _custom_providers = []
+            for _cp_entry in _custom_providers:
+                if not isinstance(_cp_entry, dict):
+                    continue
+                _cp_url = (_cp_entry.get("base_url") or "").rstrip("/")
+                if _cp_url and _cp_url == self.base_url.rstrip("/"):
+                    _cp_models = _cp_entry.get("models", {})
+                    if isinstance(_cp_models, dict):
+                        _cp_model_cfg = _cp_models.get(self.model, {})
+                        if isinstance(_cp_model_cfg, dict):
+                            _cp_ctx = _cp_model_cfg.get("context_length")
+                            if _cp_ctx is not None:
+                                try:
+                                    _config_context_length = int(_cp_ctx)
+                                except (TypeError, ValueError):
+                                    pass
+                    break
         
         # Select context engine: config-driven (like memory providers).
         # 1. Check config.yaml context.engine setting