Test heartbeat cleanup contract

[kojix2]

No description provided.

[Copilot]

Pull request overview

Adds coverage for the session heartbeat cleanup behavior and clarifies the adapter contract around heartbeat shutdown.

Changes:

• Extends the session close resource cleanup test to assert heartbeat thread start and completion.
• Adds heartbeat lifecycle tracking to the test session adapter.
• Updates the heartbeat setup comment to describe the cleanup contract.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated no comments.

File	Description
test/iruby/session_test.rb	Verifies heartbeat lifecycle completion during Session#close.
lib/iruby/session.rb	Clarifies the heartbeat adapter cleanup expectation in a comment.
lib/iruby/session_adapter/test_adapter.rb	Tracks heartbeat start/finish state for tests.

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