test(claude_code): reject empty stdin_input symmetric to prompt

run_claude validated empty prompt strings but silently accepted
stdin_input="", letting an empty stdin reach the subprocess and surface
as a confusing CLI failure instead of a clear ValueError.

tests/claude_code/_driver_unit_tests/test_cli_driver.py

@@ -301,6 +301,15 @@ def test_run_claude_validates_required_params():
             api_key="k",
             runner=runner,
         )
+    with pytest.raises(ValueError, match="stdin_input"):
+        run_claude(
+            prompt=None,
+            stdin_input="",
+            model="m",
+            base_url="http://x",
+            api_key="k",
+            runner=runner,
+        )
     with pytest.raises(ValueError, match="model"):
         run_claude(
             prompt="hi",

tests/claude_code/cli_driver.py

@@ -125,6 +125,8 @@ def run_claude(
         raise ValueError("must supply only one of `prompt` or `stdin_input`, not both")
     if prompt is not None and not prompt:
         raise ValueError("prompt must be a non-empty string when provided")
+    if stdin_input is not None and not stdin_input:
+        raise ValueError("stdin_input must be a non-empty string when provided")
     if not model:
         raise ValueError("model must be a non-empty string")
     if not base_url: