fix(config): coerce none/null empty for Optional env fields

Closes #1899.

None-sentinel coercion for Optional[str] was unreachable because the
str converter never raises and ran before the NoneType branch.

(cherry picked from commit 02b8aebe33)

gpt_researcher/config/config.py

@@ -260,16 +260,19 @@ class Config:
         args = get_args(type_hint)
 
         if origin is Union:
-            # Handle Union types (e.g., Union[str, None])
+            # Handle Union types (e.g., Union[str, None] / Optional[str]).
+            # Check the None sentinel BEFORE non-None args: for Optional[str],
+            # str conversion never raises, so looping str-first permanently
+            # shadowed the none/null/"" → None branch (see issue #1899).
+            if type(None) in args and env_value.lower() in ("none", "null", ""):
+                return None
             for arg in args:
                 if arg is type(None):
-                    if env_value.lower() in ("none", "null", ""):
-                        return None
-                else:
-                    try:
-                        return Config.convert_env_value(key, env_value, arg)
-                    except ValueError:
-                        continue
+                    continue
+                try:
+                    return Config.convert_env_value(key, env_value, arg)
+                except ValueError:
+                    continue
             raise ValueError(f"Cannot convert {env_value} to any of {args}")
 
         if type_hint is bool:

tests/test_convert_env_value_optional.py

@@ -0,0 +1,25 @@
+"""Regression tests for Optional[str] env coercion (issue #1899)."""
+from typing import Optional, Union
+
+import pytest
+
+from gpt_researcher.config.config import Config
+
+
+@pytest.mark.parametrize(
+    "raw",
+    ["none", "null", "", "NONE", "Null"],
+)
+def test_optional_str_coerces_none_sentinels(raw):
+    assert Config.convert_env_value("AGENT_ROLE", raw, Union[str, None]) is None
+    assert Config.convert_env_value("AGENT_ROLE", raw, Optional[str]) is None
+
+
+def test_optional_str_preserves_real_values():
+    assert Config.convert_env_value("AGENT_ROLE", "researcher", Optional[str]) == "researcher"
+    assert Config.convert_env_value("AGENT_ROLE", "0", Union[str, None]) == "0"
+
+
+def test_optional_int_still_coerces_and_parses():
+    assert Config.convert_env_value("SEED", "none", Optional[int]) is None
+    assert Config.convert_env_value("SEED", "42", Optional[int]) == 42