sftp_sensor: fixing resource management with sensor (apache#40022)

closes: apache#39922

Summary
When a user tries to use the SFTPSensor operator with deferrable=True, using path/newer_than, it will open a connection and remain open, the reason is because of method get_mod_time in opening a sftp connection but not closing it afterward.

As part of this change, we are closing the connection.

airflow/providers/sftp/hooks/sftp.py

@@ -549,7 +549,7 @@ async def get_files_and_attrs_by_pattern(
         matched_files = [file for file in files_list if fnmatch(str(file.filename), fnmatch_pattern)]
         return matched_files
 
-    async def get_mod_time(self, path: str) -> str:
+    async def get_mod_time(self, path: str) -> str:  # type: ignore[return]
         """
         Make SFTP async connection.
 
@@ -558,13 +558,17 @@ async def get_mod_time(self, path: str) -> str:
 
         :param path: full path to the remote file
         """
-        ssh_conn = await self._get_conn()
-        sftp_client = await ssh_conn.start_sftp_client()
+        ssh_conn = None
         try:
+            ssh_conn = await self._get_conn()
+            sftp_client = await ssh_conn.start_sftp_client()
             ftp_mdtm = await sftp_client.stat(path)
             modified_time = ftp_mdtm.mtime
             mod_time = datetime.datetime.fromtimestamp(modified_time).strftime("%Y%m%d%H%M%S")  # type: ignore[arg-type]
             self.log.info("Found File %s last modified: %s", str(path), str(mod_time))
             return mod_time
         except asyncssh.SFTPNoSuchFile:
             raise AirflowException("No files matching")
+        finally:
+            if ssh_conn:
+                ssh_conn.close()

airflow/providers/sftp/sensors/sftp.py

@@ -111,6 +111,19 @@ def poke(self, context: Context) -> PokeReturnValue | bool:
                 _newer_than = convert_to_utc(self.newer_than)
                 if _newer_than <= _mod_time:
                     files_found.append(actual_file_to_check)
+                    self.log.info(
+                        "File %s has modification time: '%s', which is newer than: '%s'",
+                        actual_file_to_check,
+                        str(_mod_time),
+                        str(_newer_than),
+                    )
+                else:
+                    self.log.info(
+                        "File %s has modification time: '%s', which is older than: '%s'",
+                        actual_file_to_check,
+                        str(_mod_time),
+                        str(_newer_than),
+                    )
             else:
                 files_found.append(actual_file_to_check)