fix(room service): enable the thread subscriptions extension based on server support

[bnjbvr]

Otherwise, when running against a server that doesn't support thread subscriptions, the client may falsely think that its thread subscriptions are not outdated (when receiving an empty, defaulted ThreadSubscriptions section in the SSS response). As a result, it would always use the store, not find most thread subscriptions, and conclude "nope, you haven't subscribed to any thread", which is wrong.

(Looking for support of MSC4306 is close to what we want, which would be support of MSC4308 itself, coming as part of element-hq/synapse#18695, and that will land despite MSC4306 being already advertised, but that's good enough, since it's unlikely many servers have enabled it by default yet.)

[codecov]

Codecov Report

❌ Patch coverage is 41.66667% with 7 lines in your changes missing coverage. Please review.
✅ Project coverage is 88.87%. Comparing base (472b934) to head (ae4ce6e).
⚠️ Report is 3 commits behind head on main.
✅ All tests successful. No failed tests found.

Files with missing lines	Patch %	Lines
crates/matrix-sdk-ui/src/room_list_service/mod.rs	41.66%	6 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #5618      +/-   ##
==========================================
- Coverage   88.88%   88.87%   -0.01%     
==========================================
  Files         348      348              
  Lines       96437    96443       +6     
  Branches    96437    96443       +6     
==========================================
- Hits        85718    85715       -3     
- Misses       6682     6689       +7     
- Partials     4037     4039       +2     

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[codspeed-hq]

CodSpeed Performance Report

Merging #5618 will improve performances by 48.07%

_{Comparing bnjbvr/gate-tsub-extension-on-server-feature (ae4ce6e) with main (472b934)}

Summary

⚡ 1 improvements
✅ 48 untouched benchmarks

Benchmarks breakdown

	Benchmark	BASE	HEAD	Change
⚡	Restore session [memory store]	403.3 ms	272.4 ms	+48.07%

[Hywan]

Looks good to me. Thanks! Would love to get a test for that, it seems sensitive, thought?