[2.x] Fix realtime extender load order for tags & sticky (TypeError: not a constructor)

[imorland]

Fixes the flarum-tags failed to initialize / TypeError: mt(...) is not a constructor error reported in https://discuss.flarum.org/d/39359 when flarum/tags and flarum/realtime are enabled together.

Root cause

flarum/realtime registers its Realtime JS extender on the export registry at module-evaluation time (flarum.reg.add('flarum-realtime', 'forum/extenders/Realtime', ...)), and consumer extensions read it back via flarum.reg.get(...). The combined forum.js concatenates extensions in the order returned by ExtensionManager::resolveExtensionOrder(), so realtime must be ordered before any consumer — otherwise the consumer's reg.get runs before realtime's reg.add and instantiates undefined.

With no dependency edge, that order falls back to reverse-alphabetical by id (Kahn's output is reversed). Extensions whose id sorts after flarum-realtime are therefore emitted before it — the broken order. That's flarum-tags and flarum-sticky. The other realtime consumers (flags, likes, lock, messages) already declare flarum/realtime as an optional dependency, which is why they were unaffected.

Fix

Declare flarum/realtime as an optional dependency on the two consumers that were missing it, so realtime is always ordered ahead of them:

• tags — added optional-dependencies: ["flarum/realtime"] (had none)
• sticky — added flarum/realtime to its existing list (also normalised flarum-tags → flarum/tags)

Tests

Added a regression test to ExtensionDependencyResolutionTest proving the ordering: without the edge, [tags, realtime] resolves to the broken order; with it, realtime is placed first.