fix: resolve all invalid-return-type ty diagnostics across codebase

Widen return type annotations to match actual control flow, add
unreachable assertions after retry loops ty cannot prove terminate,
split ambiguous union returns (auth.py credential pool), and remove
the AIOHTTP_AVAILABLE conditional-import guard from api_server.py.

agent/credential_pool.py

@@ -29,6 +29,7 @@ from hermes_cli.auth import (
     _save_auth_store,
     _save_provider_state,
     read_credential_pool,
+    read_provider_credentials,
     write_credential_pool,
 )
 
@@ -321,7 +322,7 @@ def get_custom_provider_pool_key(base_url: str) -> Optional[str]:
 
 def list_custom_pool_providers() -> List[str]:
     """Return all 'custom:*' pool keys that have entries in auth.json."""
-    pool_data = read_credential_pool(None)
+    pool_data = read_credential_pool()
     return sorted(
         key for key in pool_data
         if key.startswith(CUSTOM_POOL_PREFIX)
@@ -1303,7 +1304,7 @@ def _seed_custom_pool(pool_key: str, entries: List[PooledCredential]) -> Tuple[b
 
 def load_pool(provider: str) -> CredentialPool:
     provider = (provider or "").strip().lower()
-    raw_entries = read_credential_pool(provider)
+    raw_entries = read_provider_credentials(provider)
     entries = [PooledCredential.from_dict(provider, payload) for payload in raw_entries]
 
     if provider.startswith(CUSTOM_POOL_PREFIX):