#1525 — Add endpoint_type field to checkpoint telemetry

- New TelemetryReporter.classifyEndpoint(url) public method and
  EndpointType inner class with LOCALHOST, PRIVATE_NETWORK, REMOTE,
  UNKNOWN constants.
- buildPayload overload accepting endpoint_type threads it into the
  JSON payload; sendPing classifies the configured URL once and reuses
  the classification inside the async task.
- 20 JUnit 5 tests cover localhost (hostname, IPv4, 127/8, bracketed
  IPv6, 0.0.0.0, *.localhost), RFC1918 (10.x, 192.168.x, 172.16-31
  plus 172.15/172.32 boundary), link-local, hostname suffixes (.local,
  .internal, .lan, .intranet), public hostnames, public IPv4, empty,
  null, and malformed inputs, case insensitivity, and an explicit
  leak test that asserts the serialized payload contains no URL
  fragments.
- IPv6 hostname handling: URI.getHost() returns "[::1]" with brackets
  in some JVM versions; we strip them before comparison.

Version bump:
- pom.xml: 5.1.0 → 5.2.0