The POST /chat/sessions/:id/messages writer (and cancel + isGenerating
enrichment) resolved its per-project ChatManager/ChatStore via
getOrCreateProjectStore, while the GET reader resolves via the engine-aware
resolveProjectChatContext. When those resolved to different store instances,
a sent message persisted to one store but the reload read from another, so
regular chat (ChatView) messages vanished after leaving and returning.
Quick Chat masked it by keeping its thread warm in memory (no server reload).
resolveScopedChatManager now resolves through resolveProjectChatContext so
writer and reader always share one store. Updated multi-project routing tests
to the corrected invariant and added an engine-aware regression test.
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