Ensure serverHeartbeatEvent is sent before opening a connection

[rozza]

Added Heartbeat prose test
Migrated DefaultServerMonitorSpecification to JUnit 5.

JAVA-5230

[Copilot]

Pull Request Overview

This PR migrates the legacy Groovy DefaultServerMonitorSpecification to JUnit 5 tests and refines the heartbeat-started event emission in DefaultServerMonitor.

• Introduced DefaultServerMonitorTest.java with new JUnit 5 tests covering success, failure, and ordering of heartbeat events before socket connect.
• Removed the old DefaultServerMonitorSpecification.groovy.
• Updated DefaultServerMonitor.java to add a VisibleForTesting accessor and de-duplicate serverHeartbeatStarted events using a new flag.

Reviewed Changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 2 comments.

File	Description
driver-core/src/test/unit/com/mongodb/internal/connection/DefaultServerMonitorTest.java	Added JUnit 5 tests for heartbeat events and ordering before connection open
driver-core/src/test/unit/com/mongodb/internal/connection/DefaultServerMonitorSpecification.groovy	Removed legacy Groovy-based spec after migrating tests to JUnit 5
driver-core/src/main/com/mongodb/internal/connection/DefaultServerMonitor.java	Added getServerMonitor() for testing, alreadyLoggedHeartBeatStarted flag, and refined heartbeat-started logic

Comments suppressed due to low confidence (2)

driver-core/src/main/com/mongodb/internal/connection/DefaultServerMonitor.java:161

• [nitpick] Consider renaming 'alreadyLoggedHeartBeatStarted' to 'alreadyLoggedHeartbeatStarted' to follow consistent camelCase for the term 'heartbeat'.

private volatile boolean alreadyLoggedHeartBeatStarted = false;

driver-core/src/main/com/mongodb/internal/connection/DefaultServerMonitor.java:224

• Add a unit test that simulates multiple heartbeat cycles to verify the 'alreadyLoggedHeartBeatStarted' flag behavior and ensure only one 'ServerHeartbeatStartedEvent' is emitted per cycle.

boolean shouldStreamResponses = shouldStreamResponses(currentServerDescription);

[katcharov]

This test method corresponds to https://github.com/mongodb/specifications/pull/1483/files#diff-af358d9f2fc605678592489c97e6d557b5fa0fcb447f6619d503dc7a62c1ee71R268

But what do the other methods correspond to? Should this method be part of a ...ProseTest.java class?