Don't send datagrams combined with PMTUD packets since lost datagrams…

[jesup]

… can't be retried

[Copilot]

Pull request overview

This PR prevents QUIC datagrams from being included in PMTUD (Path MTU Discovery) probe packets. Since datagrams are unreliable and cannot be retried, including them in probe packets risks silent data loss when middleboxes drop oversized probes.

Changes:

• Added an is_pmtud_probe flag to write_appdata_frames that skips datagram frame writing when the current packet is a PMTUD probe.
• Extracted the PMTUD probe condition into a reusable is_pmtud_probe local variable, passed through to the frame-writing logic.
• Added a test verifying datagrams are excluded from PMTUD probes and sent in subsequent normal packets.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
neqo-transport/src/connection/mod.rs	Adds is_pmtud_probe parameter to write_appdata_frames and skips datagram writing when it's a probe packet
neqo-transport/src/connection/tests/datagram.rs	New test datagram_not_in_pmtud_probe verifying datagrams are excluded from PMTUD probes

[mxinden]

Not sending QUIC datagrams in PMTUD probes is a good idea.

[jesup]

This PR is part of a stack of 13 bookmarks:

1. trunk()
2. users/jesup/exportkey
3. users/jesup/draining
4. users/jesup/sub_protocol
5. users/jesup/sendGroup
6. users/jesup/stats
7. users/jesup/highwatermarks
8. users/jesup/sendgroup_sendorder_tracking
9. users/jesup/stats_overhead
10. users/jesup/avoid_PMTUD ← this PR
11. users/jesup/anticipated_streams
12. users/jesup/waitUntilAvailable
13. users/jesup/atomicWrite
14. users/jesup/WT_DRAIN_SESSION

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Created with jj-stack

[github-actions]

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QUIC Interop Runner, client vs. server

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