chore: ruff cleanup — fix 7 long-standing lint findings

Mechanical, no behavior change. `ruff check src/ tests/` now passes
with zero findings.

- cli.py:147 (SIM105) — replace `try/except SynologyError/pass` around
  the cleanup logout with `contextlib.suppress(SynologyError)`.
- compose.py:271 (B007) — drop the unused `i` from the env_list
  preview-detection loop (the apply loop below still uses enumerate).
- compose.py:329 (E501) — extract `verb = "Updated" if … else "Added"`
  into a local before the return so the f-string fits in 100 cols.
- images.py:237 (E501) — extract `stopped_name = in_use_stopped[0]`
  before the return and split the message across two f-strings.
- test_auth.py:38, 127, 140 (SIM117) — combine nested `with patch(…):`
  / `with pytest.raises(…):` into single parenthesised with-statements.

236 tests pass.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

src/mcp_synology_container/modules/images.py

@@ -233,8 +233,10 @@ def register_images(mcp: FastMCP, config: AppConfig, client: DsmClient) -> None:
             )
 
         if in_use_stopped:
+            stopped_name = in_use_stopped[0]
             return (
-                f"Cannot delete '{display_name}': image is used by stopped container '{in_use_stopped[0]}'.\n"
+                f"Cannot delete '{display_name}': image is used by stopped container "
+                f"'{stopped_name}'.\n"
                 f"Delete the container first or run system_prune to clean up stopped containers."
             )