Report when no usable GID is found

[erwinzhang7]

Related to #3467.

Connection::info() scans the GID table for an IPv4-mapped GID. If none matches, the loop
simply ends: ibv_gid gid; is never assigned and never initialized, and that value is
written into src.global_identifier and used for the queue pair. The error surfaces later
from the RTR transition as errno 22, which does not point at the GID table.

This is not a rare path. Thunderbolt RDMA ports publish only a link-local IPv6 GID until the
interface has an IPv4 address, so any Thunderbolt setup hits it before configuring one:

rdma_en3:  link_layer: Thunderbolt
           GID[0] = fe80::347c:94ff:fecd:45c4      <- no IPv4-mapped GID

After the change, the same configuration reports the cause and the remedy at the point of
failure:

[jaccl] No IPv4-mapped GID for this device. Thunderbolt RDMA ports only publish one
once the interface has an IPv4 address; assign a link-local address to it, for
example: ifconfig <interface> inet 169.254.0.1 netmask 255.255.0.0 alias

Verified

Two M4 Pro Mac minis on macOS 26.5.1, connected directly over Thunderbolt 5,
RDMA enabled from Recovery OS.

• With no IPv4 on the port, stock fails at RTR with errno 22; with this change it throws
  the message above from info().
• With an IPv4 assigned, behaviour is unchanged: info() returns ::ffff:169.254.10.1 and
  a two-rank mesh runs a correct all_sum.
• Checked on an active port. An inactive one fails earlier, in
  allocate_protection_domain, and never reaches this code.