Replace live URL embedded-image tests with deterministic local URL coverage

[bbottema]

The embedded-image URL auto-resolution tests currently depend on https://www.simplejavamail.org being reachable and trusted by the active JDK. That makes the tests vulnerable to local TLS interception, stale Java trust stores, public site availability, and CI networking differences.

Replace the live-site assumptions with deterministic local URL coverage. The tests should still exercise URL-based embedded image resolution, base URL joining, inside/outside base URL checks, and allowingEmbeddedImageOutsideBaseUrl(...), but without starting a server or relying on public HTTPS.

[bbottema]

Fixed by f11e413c.

The embedded-image URL auto-resolution tests no longer call https://www.simplejavamail.org. They now exercise the same absolute URL, relative URL, base URL, and outside-base behavior through deterministic local file: URLs, and the URL resolver now handles authority-less file: base URLs correctly.

[bbottema]

Released in Simple Java Mail v9.0.0, which is now available from Maven Central:

https://repo1.maven.org/maven2/org/simplejavamail/simple-java-mail/9.0.0/

Release notes:
https://github.com/bbottema/simple-java-mail/releases/tag/9.0.0