[Server] add a reference handler setter to server builder (#265)

* 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

CHANGELOG.md

@@ -7,6 +7,7 @@ All notable changes to `mcp/sdk` will be documented in this file.
 
 * Add built-in authentication middleware for HTTP transport using OAuth
 * Add client component for building MCP clients
+* Add `Builder::setReferenceHandler()` to allow custom `ReferenceHandlerInterface` implementations (e.g. authorization decorators)
 
 0.4.0
 -----