fix(ui): long-poll room list sync after initial request

[TigerInYourDream]

Problem

After the initial room-list sync, RoomListService continues sending timeout=0 requests because SettingUp, Recovering, and Running (before fully loaded) all forced immediate responses. This creates a tight polling loop when idle — the server returns empty responses instantly, and the client re-sends the same pos right away.

Fix

Only force timeout=0 for State::Init. All post-init states use PollTimeout::Default, letting the server long-poll when idle. If the server has pending changes, it can still respond immediately regardless of the timeout value.

Test

Added a regression test verifying the second sync request carries timeout=30000 instead of timeout=0.

• Public API changes documented in changelogs (optional)

[TigerInYourDream]

Reproduction context

• Local homeserver implementing MSC4186
• Robrix as the client, with matrix-sdk-ui driving the room-list sync

Observed symptom

After the initial sync completed and the client was idle, the homeserver kept receiving a continuous stream of requests like:

/_matrix/client/unstable/org.matrix.simplified_msc3575/sync?pos=539&timeout=0

with the same pos repeating. The loop is driven by the room-list timeout policy: all post-init states were forcing timeout=0, so the server had no choice but to respond immediately, and the client would re-send right away.

[codspeed-hq]

Merging this PR will not alter performance

✅ 50 untouched benchmarks

---

_{Comparing TigerInYourDream:fix/room-list-long-poll-after-initial-sync (c47fa7e) with main (c708ed1)}

[codecov]

Codecov Report

❌ Patch coverage is 97.22222% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 89.77%. Comparing base (d64c990) to head (c47fa7e).
⚠️ Report is 699 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	97.22%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #6361      +/-   ##
==========================================
+ Coverage   88.93%   89.77%   +0.83%     
==========================================
  Files         357      357              
  Lines       99195    98818     -377     
  Branches    99195    98818     -377     
==========================================
+ Hits        88221    88713     +492     
+ Misses       6991     6620     -371     
+ Partials     3983     3485     -498     

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