fix(frontend): default event_delivery to STREAMING, not POLLING

[BrenoMafra13]

Fixes #12291: Frontend was hardcoding POLLING as default, ignoring server's LANGFLOW_EVENT_DELIVERY config (which defaults to streaming).

Changes:

• Updated default from EventDeliveryType.POLLING to EventDeliveryType.STREAMING
• Affects buildFlowVertices, useGetConfig, and utility store initialization
• Updated corresponding test expectations
• Fallback to POLLING only when server doesn't support streaming

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