Add lightweight playback, loading, and configuration tests

[nextinfinity]

Summary

• Add JUnit Jupiter and Mockito to Gradle test/check.
• Test queue/control transitions, end reasons, notification-failure resilience, search fallback, playlist selection, failure cleanup, voice policy, and configuration parsing.
• Add small seams: injectable loader/activity callback and pure environment parsing separate from YouTube initialization.
• Add a separate Test CI job gating image build/publication.
• Document scope and conventions in TESTING.md.

Design

Three focused test classes with parameterized cases, not a broad integration suite. Manually deliver callbacks and assert application decisions; no Discord connection, audio requests, OAuth, or bot startup. Production constructors retain their existing defaults. Parsed settings redact credentials in their string representation.

Validation

• JDK 25: sh ./gradlew --no-daemon clean check shadowJar passed.
• 31 test cases across three classes, including parameterized rows.
• git diff --check passed.
• Live Discord/YouTube smoke testing remains manual.