WW-5647 Use ConcurrentHashMap for XSLT template cache

[arunmanni-ai]

The XSLT template cache in XSLTResult uses a plain HashMap with an
unsynchronized get() outside the synchronized block. This can produce
stale reads or internal corruption under concurrent access.

This patch:

• Replaces HashMap with ConcurrentHashMap for thread-safe reads
• Adds double-check inside the synchronized block to avoid
  redundant template compilation
• Removes the static initializer in favour of inline initialization