fixes related to room v12 and sytest

[FrenchGithubUser]

Discovered while writing matrix-org/sytest#1429

Those might be split into their separate PRs if necessary.

• Fix third-party (3pid) invites over federation failing intermittently in version 12 rooms, whose room IDs no longer encode a server name.
• Fix /createRoom intermittently failing with a 500 error in version 12 rooms when the same user creates several rooms at once, due to colliding room IDs.

Pull Request Checklist

• Pull request is based on the develop branch
• Pull request includes a changelog file. The entry should:
  • Be a short description of your change which makes sense to users. "Fixed a bug that prevented receiving messages from other servers." instead of "Moved X method from EventStore to EventWorkerStore.".
  • Use markdown where necessary, mostly for code blocks.
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• Code style is correct (run the linters)

[Copilot]

Pull request overview

This PR addresses two intermittent failures introduced/observed with room version 12 (MSC4291-style) rooms: (1) room ID collisions during concurrent /createRoom calls, and (2) third-party invite forwarding over federation incorrectly treating v12 room IDs as destination server names.

Changes:

• Add retry logic for MSC4291 room creation when the derived room ID collides, by perturbing origin_server_ts.
• Fix 3PID invite forwarding destination selection to avoid interpreting domainless v12 room IDs as federation destinations.
• Add a Towncrier newsfragment documenting both bugfixes.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 3 comments.

File	Description
synapse/handlers/room.py	Retries MSC4291 create-event generation on room ID collisions to avoid intermittent /createRoom 500s under concurrency.
synapse/handlers/federation.py	Adjusts 3PID invite forwarding destinations so v12+ room IDs without domains don’t trigger invalid-destination failures.
changelog.d/19898.bugfix	Adds release note entries for the two v12-related bugfixes.

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[FrenchGithubUser]

this mirrors the existing _generate_and_create_room_id, which swallows all StoreErrors the same way; distinguishing collisions from real errors is worth doing but as a separate cleanup to both