refactor(#158): collapse NotificationServiceImpl to a single constructor

[dfcoffin]

Follow-up cleanup to #158 (PR #163).

The #158 sender migration introduced two constructors on NotificationServiceImpl — a production one (built RestClient internally) and a test-seam one (injected a pre-built RestClient). That shape was the footgun that briefly turned CI red: with two constructors and neither @Autowired, Spring reported "No default constructor found" and every full-context @SpringBootTest in common failed. The fix at the time was to add @Autowired — but the cleaner answer is to not have two constructors at all.

Change

• Single constructor injecting the autoconfigured RestClient.Builder, building the client once. As the sole constructor, Spring selects it for injection with no @Autowired needed — the disambiguation hazard is gone.
• Test passes its own RestClient.builder() (still points the JDK HttpServer stub at the service); no test-only production constructor remains.

Why RestClient.Builder (not internal RestClient.create())

RestClientAutoConfiguration always provides a prototype RestClient.Builder when spring-web is on the classpath, so injecting it is safe in every context and keeps the client configurable/test-injectable through one path — which was the only reason the second constructor existed.

Verified locally: DataCustodianApplicationH2Test boots the full context, and the codec + producer wire-contract tests pass.