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

cli.py

@@ -4710,10 +4710,10 @@ class HermesCLI:
             user_provs = None
             custom_provs = None
             try:
-                from hermes_cli.config import load_config
+                from hermes_cli.config import get_compatible_custom_providers, load_config
                 cfg = load_config()
                 user_provs = cfg.get("providers")
-                custom_provs = cfg.get("custom_providers")
+                custom_provs = get_compatible_custom_providers(cfg)
             except Exception:
                 pass