Support MSC4446: allow moving fully read markers backwards

[SpiritCroc]

MSC4446

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• Pull request is based on the develop branch
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[Copilot]

Pull request overview

This PR implements MSC4446 support in Synapse, allowing clients to move the m.fully_read marker backwards when explicitly opted-in and when the experimental feature flag is enabled.

Changes:

• Add an experimental config flag (msc4446_enabled) and expose it via /_matrix/client/versions as an unstable feature.
• Extend the read-marker and receipt endpoints to accept com.beeper.allow_backward (boolean) to allow backwards movement for fully-read markers when enabled.
• Add client REST tests covering default (forwards-only) behavior, opt-in backwards behavior, and invalid usage.

Reviewed changes

Copilot reviewed 8 out of 8 changed files in this pull request and generated 2 comments.

Show a summary per file

File	Description
tests/rest/client/test_receipts.py	Adds tests for backwards movement behavior via the receipts endpoint, including feature-flag gating and invalid usage.
tests/rest/client/test_read_marker.py	Adds tests for backwards movement behavior via /read_markers, plus versions endpoint tests for the unstable feature flag.
synapse/rest/client/versions.py	Advertises com.beeper.msc4446 in unstable_features based on config.
synapse/rest/client/receipts.py	Parses/validates com.beeper.allow_backward (when enabled) and forwards it to the read-marker handler for fully-read receipts.
synapse/rest/client/read_marker.py	Parses/validates com.beeper.allow_backward (when enabled) and forwards it to the read-marker handler for m.fully_read.
synapse/handlers/read_marker.py	Adds allow_backward support to permit setting m.fully_read to an older event when opted-in.
synapse/config/experimental.py	Introduces msc4446_enabled experimental config option.
changelog.d/19663.feature	Adds Towncrier newsfragment for the feature.

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[anoadragon453]

Sorry for the glacial review time. We're burning down our backlog as a team currently.

Just a couple small things. Otherwise looks pretty good to go!

[anoadragon453]

Could you create an experimental feature issue so we can track this implementation going forwards?