Add companion endpoint for MSC4360: Sliding Sync Threads Extension

[devonh]

Implements companion endpoint from: matrix-org/matrix-spec-proposals#4360
Sliding Sync extension added in #19005

Disclaimer: I used Claude Code in a consultative manner to assist with parts of this PR.

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[MadLittleMods]

There may be some changes pending some discussions on the MSC and other Sliding Sync extension PR:

• MSC4360 Sliding Sync threads extension matrix-org/matrix-spec-proposals#4360 (comment)
• Add experimental support for MSC4360: Sliding Sync Threads Extension #19005 (comment)