test(distributed): add a process-level cluster harness

Runs local-ai as real child processes, one per frontend replica and one per
worker, against containerised infrastructure. The in-process suites cannot
express frontend-replica failure: there is no process to kill and no real HTTP
boundary between a worker and the frontend it registered with.

Assisted-by: Claude Opus 5 [claude-code]
Signed-off-by: Ettore Di Giacinto <mudler@localai.io>
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