* feat: add setReferenceHandler to Builder
Allow consumers to provide a custom ReferenceHandlerInterface
implementation (e.g. a security-aware decorator) instead of
always using the default ReferenceHandler.
* test: add BuilderTest for setReferenceHandler
- Fluent API returns builder instance
- build() succeeds with and without custom handler
- Integration test verifies custom handler intercepts tool calls
* style: fix codestyle in BuilderTest
* style: use imports instead of inline FQCNs in BuilderTest
* docs: add changelog entry for setReferenceHandler
