Part 4: Spring Boot starter and integration tests

[heruan]

Sub-issue of #323.

Scope

Add the Spring Boot starter and end-to-end integration tests across all deployment modes.

Deliverables

• observability-kit-starter module: Spring Boot auto-configuration + @ConfigurationProperties("vaadin.observability"), so a Boot application is instrumented by adding a single dependency.
• Integration-test applications verifying metrics end-to-end: a standalone (servlet) deployment, a plain-Spring deployment, and a Spring Boot application exposing metrics via Prometheus.

Dependencies

Builds on the core (#324) and plain-Spring (Part 3) modules.