[iris] Remove heartbeat log forwarding dead code

[claude]

Workers push logs directly via LogPusher; the heartbeat-piggybacked
log_entries path on WorkerTaskStatus was never populated by current
workers. Remove the forwarding block in WorkerProvider, the controller
wiring for it, the proto field (reserved 9), and the three associated
tests.

Fixes #4692

[rjpower]

@claude rebase to main pls

[claude]

Claude finished @rjpower's task in 5m 42s —— View job

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Rebase to main

• Fetch latest main
• Rebase branch onto origin/main (cherry-picked onto fresh main, resolved 2 conflicts in controller.py and test_heartbeat.py)
• Run pre-commit checks — all passed
• Run tests — all 8 remaining heartbeat tests pass
• Force push updated branch

Done. The branch has been rebased onto current main. The conflicts were in controller.py (log service setup was refactored on main to use RPC-based LogPusher instead of _InProcessLogPusher) and test_heartbeat.py (imports). Resolved by applying the PR's intent (remove WorkerProvider from log pusher wiring + delete the three dead-code tests) on top of main's current code.
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