Improve ARP buffer allocation sizing and ICMPv6 NS validation ordering

[patrzhan]

Description

Two improvements to buffer handling:

1. FreeRTOS_ARP.c: In FreeRTOS_OutputARPRequest_Multi(), the buffer allocation now uses FreeRTOS_max_size_t() to request the larger of sizeof(ARPPacket_t) and ipconfigETHERNET_MINIMUM_PACKET_BYTES, ensuring the allocated buffer is always large enough for the subsequent padding loop.

2. FreeRTOS_ND.c: In the Neighbor Solicitation case of prvProcessICMPMessage_IPv6(), the buffer size validation is reordered to occur before accessing the xIPv6Address field.

Test Steps

1. Build and run existing unit tests — no regressions.
2. Verified ARP request path sends correctly with both BufferAllocation_1 and BufferAllocation_2.
3. Verified ICMPv6 Neighbor Solicitation processing with valid and undersized packets.

Checklist:

• I have tested my changes. No regression in existing tests.
• I have modified and/or added unit-tests to cover the code changes in this Pull Request.

Related Issue

N/A

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