ci: fix Java Javadoc aggregate so /api/java deploys

[zantvoort]

Problem

https://orm.st/api/java/index.html (and the whole /api/java/ tree) 404s — the Javadoc is never published, while /api/kotlin/* works.

Root cause

The "Generate Javadoc (aggregate)" step in docs.yml fails:

[ERROR] Creating an aggregated report for both named and unnamed modules is not possible.
[ERROR] Fix the following projects:
 - storm-compiler-plugin-2.0 … 2.4, storm-ktor, storm-ktor-test

The aggregate pulls in Kotlin modules (Ktor + the compiler-plugin variants) that have no module descriptor. javadoc:aggregate can't mix these unnamed modules with the named Java modules. Because -Dmaven.javadoc.failOnError=false is set, the step still reports success while producing no output, so the copy step deploys an empty /api/java. (Confirmed: gh-pages has no api/java directory.)

Fix

1. Exclude the non-Java modules from the aggregate -pl list: storm-ktor, storm-ktor-test, and storm-compiler-plugin-2.0…2.4. Every remaining source module has a module-info.java (all named), so aggregation succeeds. These Kotlin APIs are already published as KDoc under /api/kotlin.
2. Harden the copy guard to require index.html (not just the directory), so a future empty/failed aggregate fails the build loudly instead of silently shipping an empty /api/java.

Verification

Verified every remaining module is named (find -name module-info.java). Running a local mvn install + javadoc:aggregate with the new exclusions to confirm index.html is produced. Once merged, the push to main triggers the docs deploy; with the hardened guard, if anything is still off it will fail rather than silently 404.

[zantvoort]

✅ Validated locally: with the new exclusions, mvn install + mvn javadoc:aggregate exits 0 with no "named and unnamed modules" error and produces target/reports/apidocs/index.html (14 st.orm packages, 33 entries). The copy step finds it at target/reports/apidocs and the hardened index.html guard passes.