fix(BA-3162): Remove agent heartbeat handling observation from audit log

[seedspirit]

resolves #6966 (BA-3162)

Checklist: (if applicable)

• Milestone metadata specifying the target backport version
• Mention to the original issue
• Installer updates including:
  • Fixtures for db schema changes
  • New mandatory config options
• Update of end-to-end CLI integration tests in ai.backend.test
• API server-client counterparts (e.g., manager API -> client SDK)
• Test case(s) to:
  • Demonstrate the difference of before/after
  • Demonstrate the flow of abstract/conceptual models with a concrete implementation
• Documentation
  • Contents in the docs directory
  • docstrings in public interfaces and type annotations

[Copilot]

Pull request overview

This PR removes HandleHeartbeatAction.spec() from the list of supported actions in the agent processors, effectively excluding agent heartbeat handling from being listed as a supported action. This aligns with the treatment of other internal system actions.

• Removes HandleHeartbeatAction from the supported_actions() method return list
• Maintains the actual heartbeat processing functionality (processor still initialized and functional)
• Follows the existing pattern where internal actions like mark_agent_exit and mark_agent_running are also excluded from supported_actions()

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[HyeockJinKim]

Shouldn't we be looking at the part where we're applying action monitors, rather than removing the spec?