fix(filesystem): clarify no-tcal fallback comment and info_msg

Address review feedback on PR #1553: the previous "Starting with the
first non-tcal file." log line was technically inaccurate when the
fallback path fired, because we then start from the last file rather
than position [2]. Split the two cases into distinct branches with
distinct log messages so the user sees exactly which case applied.

No behavioural change. Tests unchanged and still pass.

Signed-off-by: Yash Goel <yashvardhan664@gmail.com>

Author: yashhzd

ardupilot_methodic_configurator/backend_filesystem.py

@@ -773,15 +773,19 @@ def get_start_file(self, explicit_index: int, tcal_available: bool) -> str:
                 )
             return files[start_file_index]
 
+        # In the no-tcal branch the historical behaviour is files[2], i.e. skip
+        # the first two files (typically 00_default.param + 01_tcal.param). Fall
+        # back to the last available file when fewer than 3 files are present
+        # so we never crash on a small or non-standard file set.
         if tcal_available:
             start_file = files[0]
             info_msg = _("Starting with the first file.")
-        else:
-            # files[2] skips 00_default.param + 01_tcal.param when tcal is not
-            # available; fall back to the last file if fewer than 3 are present
-            # so we never crash on a small or non-standard file set.
-            start_file = files[2] if len(files) >= 3 else files[-1]
+        elif len(files) >= 3:
+            start_file = files[2]
             info_msg = _("Starting with the first non-tcal file.")
+        else:
+            start_file = files[-1]
+            info_msg = _("Fewer than three files available; starting with the last file.")
 
         last_uploaded_filename = self.__read_last_uploaded_filename()
         if last_uploaded_filename: