feat(rendezvous): add UDP punch negotiation fields

[Bia503]

Summary

Adds wire-compatible rendezvous protocol fields required for authenticated UDP hole-punch negotiation:

• TestNatRequest.nonce
• TestNatResponse.nonce
• PunchHole.udp_punch_token
• PunchHoleSent.udp_punch_token
• RegisterPk.udp_punch_token_supported

Motivation

The existing UDP candidate path does not have a server-verified completion signal.
These fields allow a rendezvous server to:

• bind UDP completion to a short-lived, single-use token;
• verify NAT test replies belong to the active request;
• negotiate the feature only with clients that explicitly support it.

All fields are additive protobuf fields. Older clients and servers ignore them and
continue using the existing TCP/relay flow.

Follow-up

Client and hbbs support will be submitted separately after this protocol change.

Summary by CodeRabbit

• New Features
  • Added support for UDP punch tokens during connection setup.
  • Added nonce exchange for NAT connectivity testing.
  • Added capability signaling for UDP punch token support.

[coderabbitai]

No actionable comments were generated in the recent review. 🎉

ℹ️ Recent review info

⚙️ Run configuration

Configuration used: defaults

Review profile: CHILL

Plan: Pro Plus

Run ID: 6e3ac6fb-fdb6-412e-b3bf-3caa3ff66b1b

📥 Commits

Reviewing files that changed from the base of the PR and between f124c0a and 3a39904.

📒 Files selected for processing (1)

• protos/rendezvous.proto

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📝 Walkthrough

Walkthrough

The rendezvous protobuf definitions add UDP punch token fields, NAT test nonces, and a registration capability flag.

Changes

Rendezvous protocol

Layer / File(s)	Summary
Rendezvous message fields protos/rendezvous.proto	PunchHole and PunchHoleSent now carry UDP punch tokens. TestNatRequest and TestNatResponse now carry nonces. RegisterPk now reports UDP punch token support.

Estimated code review effort: 1 (Trivial) | ~5 minutes

Suggested reviewers: rustdesk, 21pages

🚥 Pre-merge checks | ✅ 5

✅ Passed checks (5 passed)

Check name	Status	Explanation
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.
Title check	✅ Passed	The title clearly and concisely summarizes the addition of UDP punch negotiation fields in the rendezvous protocol.
Docstring Coverage	✅ Passed	No functions found in the changed files to evaluate docstring coverage. Skipping docstring coverage check.
Linked Issues check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.

✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests

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[Bia503]

FYI, I’m holding the follow-up client/hbbs PRs for now, since they depend on this protocol change and won’t pass CI against the current hbb_common.