[ycabled]: Add IPv6 only active-active SoC gRPC

[zjswhhh]

Why I did it

Active-active DualToR IPv6 only clusters can omit soc_ipv4. ycabled should continue to prefer IPv4 for dual-stack configs, but connect to the SoC over IPv6 when IPv4 is absent.

How I did it

• Prefer soc_ipv4 when present.
• Fall back to soc_ipv6 only when soc_ipv4 is missing.
• Strip CIDR notation before building the gRPC target.
• Format IPv6 gRPC targets as [addr]:port.
• Add ycabled tests for IPv4 preference, IPv6 fallback, and IPv6 target formatting.

How to verify it

• Run the sonic-ycabled unit tests.
• With an IPv6 only active-active mux cable config, verify ycabled opens gRPC connections using [soc_ipv6]:<port>.

[mssonicbld]

/azp run

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