Looking to debug 802.3AT PoE+ LLDP power class negotiation frames exchanged between a Non-Cisco PoE-Plus Powered device and a Cisco switch

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Is there a dissector already created for this purpose?

Wireshark has an LLDP dissector.

If so, is it able to identify/decode the power class requests / responses between a PoE PD (Powered Device) and a PSE (Power Sourcing Equipment - i.e.: a Cisco switch)?

That dissector includes support for 802.3 TLVs, including the Power Via MDI TLV.

So, if the machine on which you're running Wireshark can capture those frames, Wireshark should be able to dissect them.