Keep the missing-manifest diagnostic at debug

The legacy manifest-less layout is legal and the 3.1.2 core ships one
such library (FSTools), so the warning would be unactionable noise; a
test now pins the level.

esphome/arduino/library.py

@@ -267,9 +267,10 @@ def _bundled_library(framework_path: Path, name: str) -> ArduinoLibrary:
     elif (manifest := lib_dir / "library.properties").is_file():
         data = parse_library_properties(manifest)
     else:
-        # Visible: core libraries all ship a manifest, so an absent one
-        # most likely means a torn framework extraction
-        _LOGGER.warning("Bundled library %s has no manifest; using defaults", name)
+        # Debug, not warning: the legacy manifest-less layout is legal and
+        # the 3.1.2 core ships one such library (FSTools), so a warning
+        # would be unactionable noise on every build using it
+        _LOGGER.debug("Bundled library %s has no manifest; using defaults", name)
         data = {}
     if isinstance(data, dict):
         # Bundled manifest deps are never walked; make the skip visible

tests/unit_tests/test_arduino_library.py

@@ -4,6 +4,7 @@ from __future__ import annotations
 
 from contextlib import contextmanager
 import json
+import logging
 from pathlib import Path
 from unittest.mock import patch
 
@@ -900,6 +901,18 @@ def test_bundled_library_non_dict_manifest_skips_probes_and_raises(
         component._bundled_library(framework, "Wire")
 
 
+def test_bundled_missing_manifest_is_debug_only(
+    tmp_path: Path, caplog: pytest.LogCaptureFixture
+) -> None:
+    """The legacy manifest-less layout is legal (the core ships FSTools
+    without one), so the diagnostic must stay below warning level."""
+    framework = _make_framework(tmp_path)
+    with caplog.at_level(logging.DEBUG):
+        component._bundled_library(framework, "Wire")
+    record = next(r for r in caplog.records if "has no manifest" in r.message)
+    assert record.levelno == logging.DEBUG
+
+
 def test_bundled_corrupt_library_json_fails_by_name(tmp_path: Path) -> None:
     """A truncated bundled library.json fails with the library name and the
     clean-all hint, not a raw JSONDecodeError."""