Fix integer overflow in IPFIX exporter delta calculation

[Av1ralS1ngh]

The IPFIX exporter computes delta counts for packets and bytes by subtracting the previous counter value from the current one. The original code cast both uint64 operands to int64 before subtraction, which silently overflows when counter values exceed math.MaxInt64 (~9.2 × 10¹⁸).

This change removes the int64 casts and performs the subtraction directly in uint64 arithmetic, which is correct for monotonically increasing counters and also handles wrap-around naturally. A conditional log is emitted when the current value is less than the previous value (counter reset), preserving observability without altering the exported value.

What changed:

• pkg/agent/flowexporter/exporter/ipfix.go: replaced int64 casts with direct uint64 subtraction for packetDeltaCount, octetDeltaCount, reversePacketDeltaCount, and reverseOctetDeltaCount.

• pkg/agent/flowexporter/exporter/ipfix_overflow_test.go [NEW]: table-driven test covering normal deltas, values exceeding MaxInt64, and counter-reset scenarios.

  Fixes Integer overflow in IPFIX exporter delta calculation #7783