XB10-2731: Disable setting of 'Auto' WAN mode in XB10.

[jayantrais]

Reason for change: Disable setting of 'Auto' WAN mode in XB10.
Test Procedure: Build and Verify
Risks: medium
Priority: P1
Signed-off-by: Jayant_Rai2@comcast.com

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[Copilot]

Pull request overview

This PR updates the Ethernet WAN TR-181 setter logic to prevent selecting the "Auto" operational mode when building for the XB10 product, aligning platform capabilities with the intended configuration behavior.

Changes:

• Extends the existing platform compile-time guard to also block "Auto" mode on _XB10_PRODUCT_REQ_.
• Updates the warning log text emitted when "Auto" is rejected.

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