Skip the results email when no reports were parsed (#200)

The [smtp] (and Microsoft Graph) results email was sent unconditionally
whenever the transport was configured, so an empty run — an empty inbox,
or one where every message was invalid — still emailed a zip of
headers-only CSVs. The email step is now skipped with an INFO log when
the run produced no aggregate, failure, or SMTP TLS reports.

The regression test was verified to fail against the unfixed code
(send_email was called once with a headers-only zip). Six existing tests
that asserted on the email path with all-empty mocked results now feed a
real parsed sample aggregate report through the actual zip/CSV-building
code instead.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

docs/source/usage.md

@@ -527,6 +527,11 @@ The full set of configuration options are:
   - `aggregate_topic` - str: The Kafka topic for aggregate reports
   - `failure_topic` - str: The Kafka topic for failure reports
 - `smtp`
+
+  The results email is only sent when at least one aggregate, failure,
+  or SMTP TLS report was parsed during the run; an empty run (e.g. an
+  empty inbox) skips the email instead of sending headers-only CSVs.
+
   - `host` - str: The SMTP hostname. Required unless `[msgraph]` is
     configured, in which case omitting it sends the summary via
     Microsoft Graph instead — see "Sending the summary email via