fix: resolve javadoc redirects before resource lookup - #1213

Summary

Resolve redirects on an external Javadoc base URL before looking up its package-list or element-list. This supports sites that redirect a base URL to an index page instead of forwarding deep resource paths to the corresponding resource.

Changes

• Resolve the final base URL before constructing package and module resource URLs.
• Preserve the resolved base URL for generated links.
• Add a local HTTP regression test covering a base URL redirect to an index page.

Testing

• mvn -pl maven-plugin-tools-api -Dtest=JavadocSiteTest -Dspotless.apply.skip=true -Dspotless.check.skip=true test — PASS, 27 tests.
• mvn -pl maven-plugin-tools-api -am -DskipTests -Dspotless.apply.skip=true -Dspotless.check.skip=true verify — PASS.
• mvn -pl maven-plugin-tools-api -Dspotless.apply.skip=true -Dspotless.check.skip=true test — PASS, 62 tests.
• git diff --check — PASS.
• mvn -pl maven-plugin-tools-api spotless:apply — BLOCKED by the repository's existing Palantir formatter/JDK incompatibility (NoSuchFieldError in JCTree$JCCompilationUnit across baseline files).

Notes

• The regression test uses an in-process HTTP server and does not depend on external network availability.
• AI assistance was used during preparation; the submitting contributor reviewed the changes and test results.

Fixes #1041