Fix leaf node advertisement with no_advertise

[freakes]

Summary

Respect leafnodes.no_advertise when advertising leaf-node listener URLs.

Previously, the server always added its own leaf listener address to the
advertised leaf-node URL set. As a result, connected leaf nodes could learn
the hub address and later try to reconnect to it even when no_advertise
was enabled.

Avoid adding the local leaf listener URL when no_advertise is set.

This also means a legitimate initial leaf INFO may contain no
LeafNodeURLs. Update the solicited leaf handshake validation so an
InfoOnConnect INFO without advertised leaf URLs is not incorrectly
classified as a wrong-port connection.

Tests

Added regression coverage for:

• a hub configured with leafnodes.no_advertise=true not leaking its leaf
  listener into the leaf's reconnect URL set
• accepting a legitimate InfoOnConnect leaf INFO without LeafNodeURLs
• preserving existing wrong-port detection behavior

Verified with:

go test ./server -run '^TestLeafNodeNoAdvertiseDoesNotAdvertiseListener$' -count=20
go test ./server -run '^TestLeafNodeInfoOnConnectWithoutLeafNodeURLsAccepted$' -count=20
go test ./server -run '^TestLeafNodeRemoteWrongPort$' -count=20
go test ./server -run 'LeafNode.*WrongPort|LeafNodeNoAdvertise' -count=10

Resolves #8367

[chatgpt-codex-connector]

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[chatgpt-codex-connector]

Clear discovered leaf URLs when advertisements become empty

When leafnodes.no_advertise suppresses the server's own URL, leafURLsMap can now legitimately become empty after the last routed server that advertised a leaf URL disconnects. removeLeafNodeURL will send an async INFO with no leafnode_urls, but soliciting leaves only refresh discovered URLs when the incoming INFO has a non-empty URL list, so they keep stale discovered peers instead of falling back to the configured remote. In a no-advertise hub cluster, this leaves leaf nodes reconnecting to removed peers; empty async advertisements need to be applied as the current URL set too.

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