Name the missing-table state for the backstop

Still the backstop's business, but a clean message beats a bare
FileNotFoundError for the one impossible state most likely to appear
in a report.

esphome/espidf/size_summary.py

@@ -53,8 +53,11 @@ def _find_app_partition_size(partitions_csv: Path) -> int | None:
     naive "prefer factory" rule would pick the wrong row. No qualifying
     row is legitimate absence (None); a build cannot succeed with a
     missing or malformed table (gen_esp32part consumes it first), so
-    those states belong to the backstop.
+    those states belong to the backstop -- the missing-file raise just
+    names that one cleanly.
     """
+    if not partitions_csv.is_file():
+        raise ValueError(f"partitions.csv not found at {partitions_csv}")
     for row in csv.reader(partitions_csv.read_text(encoding="utf-8").splitlines()):
         cells = [c.strip() for c in row]
         if not cells or cells[0].startswith("#") or len(cells) < 5:

tests/unit_tests/test_size_summary.py

@@ -305,3 +305,16 @@ def test_print_summary_corrupt_size_json_warns(
     print_summary(size_json, None)
     assert capsys.readouterr().out == ""
     assert "Skipping size summary" in caplog.text
+
+
+def test_print_summary_missing_partitions_named_in_backstop(
+    tmp_path: Path,
+    capsys: pytest.CaptureFixture[str],
+    caplog: pytest.LogCaptureFixture,
+) -> None:
+    """A vanished table is an impossible post-build state; the backstop
+    reports it by name instead of a bare FileNotFoundError."""
+    size_json = _write_size_json(tmp_path, _dram_size_data())
+    print_summary(size_json, tmp_path / "nope.csv")
+    assert "Flash:" not in capsys.readouterr().out
+    assert "partitions.csv not found" in caplog.text