Fix: ipv4/ipv6_contactable metrics never updated

[ackintosh]

Description

• Fix METRICS.ipv4_contactable and METRICS.ipv6_contactable never being updated due to an inverted condition in Config::build()
• Add regression tests for both metrics
• Fix cargo clippy errors
  • Suppress clippy::collapsible_match with explanatory comments where PendingEntry::value()requires&mut self` and cannot be used in a pattern guard
  • (Edit) The clippy fixes have been split into Fix clippy errors #301

Background

Config::build() contained an inverted boolean condition:

// Intended: disable auto-NAT when enr_update is false
if self.config.enr_update {   // ← should be `if !self.config.enr_update`
    self.config.auto_nat_listen_duration = None;
}

Because enr_update defaults to true, this condition was always true, so auto_nat_listen_duration was unconditionally overwritten with None. As a result:

• ConnectivityState never started an incoming-connection timer
• received_incoming_connection() always returned early
• METRICS.ipv4_contactable and METRICS.ipv6_contactable remained false permanently regardless of actual reachability

Notes & open questions

Change checklist

• Self-review
• Documentation updates if relevant
• Tests if relevant

[jxs]

Hi, Akihito, thanks for looking into this! Can we separate the clippy PR into a separate PR? I am also wondering if we can test in a more isolated way as this test is quite complex and needs flags to not affect other tests

[jxs]

can you solve the merge conflicts @ackintosh? Should be good to go after

[ackintosh]

@jxs I've updated this PR, thanks!