Fixes #6775: Guard ExtensionLoader joins during concurrent initialization

[BobSong-dev]

Fixes #6775

Background

ExtensionLoader#getJoins() treated cachedInstances.size() matching the number of extension classes as proof that every cached Holder was ready. However, getJoin() publishes a new Holder before populating its order and value. A concurrent getJoins() call could therefore sort or return an incompletely initialized holder, causing an NPE or a null element.

Changes

• Track Holder initialization with a volatile flag published after order and value are set;
• Use the cached fast path only when every cached Holder is fully initialized;
• Add a concurrent regression test for getJoins() observing an incomplete Holder.

Verification

• .\mvnw.cmd -pl shenyu-spi test -DskipITs: 20 tests passed, 0 failures, 0 errors, 0 skipped;
• Checkstyle: 0 violations;
• git diff --check: passed;