State what the cache refresh actually writes

The comment claimed the refresh's sidecar records the CLI toolchain;
_refresh_sidecar returns early when a compile-written sidecar exists,
so the compile's toolchain stands. Reword the comment and the test
docstring to match.

esphome/__main__.py

@@ -2739,8 +2739,10 @@ def run_esphome(argv):
     )
     # An explicit CLI toolchain must run the per-platform validators; the
     # cache was validated under whatever the last compile used. Only the
-    # read is gated: the refresh below may still save the freshly
-    # validated config, and its sidecar records the resolved toolchain.
+    # read is gated: the refresh below still saves the freshly validated
+    # config. The sidecar is only written when none exists; a
+    # compile-written one keeps the compile's toolchain (the firmware on
+    # disk was built by it), which upload/logs then restore.
     cache_read_eligible = cache_write_eligible and args.toolchain is None
     if cache_read_eligible:
         from esphome.compiled_config import load_compiled_config

tests/unit_tests/test_main.py

@@ -7253,8 +7253,8 @@ def test_cli_toolchain_still_refreshes_the_validated_config_cache(
     tmp_path: Path,
 ) -> None:
     """An explicit --toolchain gates only the cache read; the freshly
-    validated config is still saved, and its sidecar records the resolved
-    toolchain for a later plain run."""
+    validated config is still saved so a later plain run keeps the fast
+    path (an existing compile-written sidecar keeps its toolchain)."""
     from esphome.__main__ import run_esphome
 
     conf = tmp_path / "device.yaml"