fix(api-errors): keep 4xx lower bound; drop redundant 500 check

The earlier `if status < 500` simplification was behaviour-changing: after
the `status >= 500` early return the upper bound is always true (CodeQL
"redundant comparison" + "unreachable code" on the `return "api"` fallback),
but dropping the `>= 400` lower bound also routed sub-400 statuses (e.g. the
`status=0` default for non-HTTP-ish errors) into `4xx_client` instead of the
generic `api` bucket. Use `if status >= 400` — equivalent to the original
`400 <= status < 500` given the preceding return, with the `api` fallback
reachable again for status < 400.

Also parenthesize the remaining sparse plan-mode reminder concatenation so
CodeQL's implicit-string-concatenation check stays quiet without splitting
the line across the rendered output.

Author: elkaix

src/pythinker_code/soul/api_errors.py

@@ -41,7 +41,7 @@ def classify_api_error(e: Exception) -> tuple[str, int | None]:
             return "auth", status_code
         if status >= 500:
             return "5xx_server", status_code
-        if status < 500:
+        if status >= 400:
             msg_lower = str(e).lower()
             if any(marker in msg_lower for marker in _CONTEXT_OVERFLOW_MARKERS):
                 return "context_overflow", status_code

src/pythinker_code/soul/dynamic_injections/plan_mode.py

@@ -225,8 +225,10 @@ def _sparse_reminder(plan_file_path: str | None = None) -> str:
     )
     parts.extend(
         [
-            "Exit only with a decision-complete plan; "
-            "record unconfirmed defaults under Assumptions.",
+            (
+                "Exit only with a decision-complete plan; "
+                "record unconfirmed defaults under Assumptions."
+            ),
             "Use AskUserQuestion to clarify user preferences "
             "when it helps you write a better plan.",
             "If the plan has multiple approaches, "