watcher: save and notify nothing for a cut-short port or TLS check (closes #185)

When shutdown cancels a check that is under way, the rest of the check
still runs with the cancelled context. The resolver already drops a
lookup the context cut short, but a cancelled connection attempt was
saved as a closed port or a failed certificate check and notified as
Port Change or TLS Failure. The watcher now drops a port or TLS check
result when its context was cancelled, the same way. The test runs a
check with the context already cancelled, using the real resolver and
the real port and TLS checkers; no query is sent and no connection is
made.

Model: opus-5-5

TODO.md

@@ -19,6 +19,8 @@ nameserver IP address changes: https://git.eeqj.de/sneak/dnswatcher/issues/105
 
 # Completed Steps
 
+- 2026-10-01: a port or TLS check that shutdown cuts short saves nothing and
+  sends no notification, as a cut-short DNS lookup already did (closes #185).
 - 2026-10-01: the client address from `X-Forwarded-For` is the last entry that
   is not a trusted proxy, not the first, which the client sets (closes #181).
 - 2026-10-01: a nameserver that does not answer is saved as `error` with the