[iris] Drop implicit SA fallback from SSH impersonation

[rjpower]

Remove the service_account parameter from ssh_impersonate_service_account()
so impersonation only happens when explicitly configured in SshConfig. Previously
the function fell back to the worker/controller service_account, which caused
metadata-mode SSH commands to include --impersonate-service-account unnecessarily,
requiring the controller SA to hold iam.serviceAccountUser on every worker SA.
Now metadata mode uses the VM's ambient identity and OS Login setups must set
ssh.impersonate_service_account explicitly.

[claude]

Claude finished @rjpower's task in 1m 9s —— View job

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PR Review: Drop implicit SA fallback from SSH impersonation

• Read changed files and understand the diff
• Check for correctness and edge cases
• Post review feedback

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Verdict: Looks good — clean, focused removal of the implicit fallback. The change is well-motivated and correctly applied across all call sites.

What this does

Removes the service_account parameter from ssh_impersonate_service_account() so it only returns an SA when ssh_config.impersonate_service_account is explicitly set. Previously, it fell back to the worker/controller SA, which caused metadata-mode SSH to include --impersonate-service-account unnecessarily.

Review notes

1. All call sites updated consistently — handles.py, controller.py, workers.py all stop passing the now-removed parameter. No stale references remain.

2. Tests updated correctly — test_gcp_vm_slice_os_login_uses_service_account_impersonation (line 478) and test_gcp_tpu_slice_os_login_resolves_user_from_service_account (line 860) now assert None instead of the old fallback SA, which matches the new behavior. The "prefers explicit" test variants are unchanged and still verify the explicit ssh_config.impersonate_service_account path works.

3. Minor observation: _service_account field is now unused on the handles — GcpStandaloneWorkerHandle._service_account and GcpVmSliceHandle._service_account are still stored and passed through (e.g., handles.py:459) but no longer read for any purpose within the handle classes themselves. If nothing external reads these fields, they could be cleaned up in a follow-up. Not a blocker.

4. _gcp_tunnel signature cleaned up — the service_account kwarg is removed from the private _gcp_tunnel function and its single caller in GcpControllerProvider.tunnel no longer passes it. Clean.

No issues found. Ship it.

[ravwojdyla]

this still needed if #4381 was merged?

[rjpower]

this is more cleaning up the path that shouldn't have been there, i'll rebase