Fix ml service memory leak

[myungjoo]

Fix memory leak in ml_service_new/ml_service_destroy loop by ensuring server->id is properly initialized and reset.

The server->id was not consistently initialized or reset, causing ml_agent_pipeline_destroy() to be skipped or called with an incorrect ID, leading to server-side pipeline resource leaks during repeated ml_service_new() and ml_service_destroy() calls.

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[cursor]

Cursor Agent can help with this pull request. Just @cursor in comments and I'll start working on changes in this branch.
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