fix: suppress HostUnreachable warnings on UDP transmit (fixes #4345)

[Abhijeet1815]

Description

Breaking Changes

Notes & open questions

Change checklist

• Self-review.
• Documentation updates following the style guide, if relevant.
• Tests if relevant.
• All breaking changes documented.
  • List all breaking changes in the above "Breaking Changes" section.
  • Open an issue or PR on any number0 repos that are affected by this breaking change. Give guidance on how the updates should be handled or do the actual updates themselves. The major ones are:
    • quic-rpc
    • iroh-gossip
    • iroh-blobs
    • dumbpipe
    • sendme

[Frando]

Can you add some more details please. When are you getting these errors? Which platform, and under which conditions?

If suppressing these errors is what is wanted on the recv path generally (I'll have to investigate more if that is the case), then this should likely happen in netwatch, similar to n0-computer/net-tools#166

Otherwise yes, it's likely fine to tone down the warnings here. But I'd like to know a bit more before.

[Abhijeet1815]

Description

This PR fixes #4345 by suppressing expected HostUnreachable warnings from noq UDP packet transmits on the send path.

Root Cause & Context:
Since PR #4299 introduced Happy Eyeballs support, iroh resolves both IPv4 and IPv6 addresses for the Relay URL and simultaneously probes them.

On hosts that have an IPv6 interface (e.g., link-local addresses, which makes if_state.have_v6 evaluate to true) but lack an actual IPv6 internet route, the QADv6 (QUIC address discovery) probe is still triggered. When noq attempts to send a UDP packet to the Relay's IPv6 address via netwatch::UdpSender, the OS (sendmsg) naturally rejects it with Os { code: 65, kind: HostUnreachable, message: "No route to host" }.

The Fix:
Previously, the underlying poll_send logic in iroh/src/socket/transports.rs would log a WARN for every packet dropped this way, resulting in spurious log spam.

Because HostUnreachable is a perfectly expected outcome on the send path when Happy Eyeballs probes partially connected network topologies, this PR downgrades HostUnreachable, NetworkUnreachable, and NetworkDown errors during packet transmission to trace! level. This gracefully drops the packets without polluting the user logs, while preserving visibility for developers debugging at a granular level.

Breaking Changes

None.

Notes & open questions

• HostUnreachable, NetworkUnreachable, and NetworkDown are explicitly handled as expected behaviors on isolated/local networks trying to dial out to the internet, and are now only traced.

Change checklist

• Self-review
• Documentation updates following the style guide, if relevant.
• Tests if relevant.
• All breaking changes documented.
  • List all breaking changes in the above "Breaking Changes" section.
  • Open an issue or PR on any number0 repos that are affected by this breaking change. Give guidance on how the updates should be handled or do the actual updates themselves. The major ones are:
    • quic-rpc
    • iroh-gossip
    • iroh-blobs
    • dumbpipe
    • sendme

[flub]

The warn log should not exist at all to be fair. Let's not do any special casing here and move the entire log to trace.

noq-udp already does appropriate logging for these.

Also, please update the description of the PR explaining the reasoning and closing the issue. This is what gets turned into the commit message. If you can not do this we'll have to close this PR.