Fix OtelInternalJavadoc diagnostic message to match the contains check

[thswlsqls]

Fixes #8589

Description

• OtelInternalJavadoc matches when the internal disclaimer is absent using javadoc.contains(...) (lines 53-56), but the @BugPattern summary said the class "doesn't end with" the disclaimer.
• Changed the summary to "doesn't contain", so the message describes what the check verifies.
• This aligns with the sibling OtelImplJavadoc, which already describes the same .contains() logic accurately ("is missing the required javadoc disclaimer").

Testing done

• Updated the three // BUG: Diagnostic contains: assertions in OtelInternalJavadocTest#positiveCases to match the new message.
• ./gradlew :custom-checks:check — passed.
• custom-checks is an unpublished build module (no maven-publish), so there is no japicmp/apidiff and no CHANGELOG entry (the message is build-time contributor-facing, not user-facing).