openflow: fix multi-part reply and drain pending msgs

[abhiramnarayana]

Fix 'Broken pipe' errors in ovs-vswitchd.log caused by the exporter closing OpenFlow connections prematurely.

• Add drainPendingMessages() helper to discard pending OVS messages
• Modify getFlowStats() to loop until OFPSF_REPLY_MORE is unset
• Add drain call to both getFlowStats() and GetAggregateStats()
• Add unit tests for multi-part handling and drain functionality

Closes: OSPRH-32382

(cherry picked from commit 38c64db)

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