test(e2e): add dump-only-sql mixed-run + CLI contract coverage

- rename dump-only flag to --dump-only-sql across docs and tests
- update backup logic: skip files/ only for DB volumes when dumps succeed; fallback to files when dumps fail
- extend e2e helpers to support dump_only_sql
- add e2e mixed-run regression test (DB dump => no files/, non-DB => files/)
- add e2e CLI/argparse contract test (--dump-only-sql present, --dump-only rejected)
- fix e2e files test to expect file backups for non-DB volumes in dump-only-sql mode and verify restore
- update changelog + README flag table

https://chatgpt.com/share/69522d9c-ce08-800f-9070-71df3bd779ae

tests/e2e/test_e2e_mariadb_no_copy.py

@@ -87,7 +87,7 @@ class TestE2EMariaDBNoCopy(unittest.TestCase):
             [(cls.db_container, cls.db_name, cls.db_user, cls.db_password)],
         )
 
-        # dump-only => no files
+        # dump-only-sql => no files
         backup_run(
             backups_dir=cls.backups_dir,
             repo_name=cls.repo_name,
@@ -95,7 +95,7 @@ class TestE2EMariaDBNoCopy(unittest.TestCase):
             databases_csv=cls.databases_csv,
             database_containers=[cls.db_container],
             images_no_stop_required=["mariadb", "mysql", "alpine", "postgres"],
-            dump_only=True,
+            dump_only_sql=True,
         )
 
         cls.hash, cls.version = latest_version_dir(cls.backups_dir, cls.repo_name)