feat: Enhance review prompt with detailed instructions and placeholders for empty sections

tests/test_reviewer_fallback.py

@@ -57,6 +57,27 @@ def test_build_prompt_includes_trigger_message() -> None:
     assert "Trigger message: @codex review security\nPlease focus auth." in prompt
 
 
+def test_build_prompt_uses_empty_placeholders_and_no_network_instruction() -> None:
+    pr = type("PR", (), {"html_url": "https://gitea.example/pr/1"})()
+    command = ParsedCommand(name="review", raw="@codex review")
+    diff_context = {"truncated": False, "changed_files": [], "diff": ""}
+    repo_cfg = RepoReviewConfig()
+
+    prompt = _build_prompt(
+        pr,
+        command,
+        diff_context,
+        repo_cfg,
+        changed_file_contents="",
+        test_output=None,
+    )
+
+    assert "You do not have internet/network access. Do not try to fetch URLs." in prompt
+    assert "Never claim that PR content is inaccessible or missing if these sections are present." in prompt
+    assert "Changed files:\n(none)" in prompt
+    assert "Unified diff:\n(empty)" in prompt
+
+
 def test_prepare_review_prompt_applies_repo_default_mode_when_command_mode_not_explicit(monkeypatch, tmp_path) -> None:
     repo_dir = tmp_path / "repo"
     repo_dir.mkdir(parents=True, exist_ok=True)