fix: show promoted queued message in chat timeline immediately

[mafredri]

Two issues caused the promoted message to never appear in the chat
timeline.

Problem 1: handlePromoteQueuedMessage discarded the ChatMessage
returned by the promote API, relying entirely on the WebSocket to
deliver it.

Problem 2 (root cause): even when the WebSocket delivered the
message (via upsertDurableMessage), the queue_update event in the
same batch called updateChatQueuedMessages, which mutated the React
Query cache. This gave chatMessagesList a new reference, triggering
the message sync effect. The effect found the promoted message in the
store but not in the (stale) REST-fetched data, classified it as a
"stale entry" (a code path designed for edit truncation), and called
replaceMessages, wiping the promoted message from the store before
it ever rendered.

Fix 1 (AgentDetail.tsx): capture the ChatMessage from the
promote response and upsert it into the store immediately, matching
the pattern in handleSend for non-queued messages.

Fix 2 (ChatContext.ts): track the fetched message array elements
across effect runs using element-level reference comparison. Only run
the hasStaleEntries / replaceMessages path when the message
objects actually changed (e.g. a refetch producing new objects from the
server), not when only an unrelated field like queued_messages caused
the query data reference to update. Element references work because
useMemo(flatMap) preserves object identity when only non-message
fields change in the page data. This avoids the ghost-on-edit-truncation
regression that a pure ID-fingerprint approach would have (IDs stay the
same after content edits, so the guard would never fire).

A regression test verifies that a WebSocket-delivered message survives
a queue_update in the same batch without being wiped by the sync
effect.

🤖 This PR was created with the help of Coder Agents, and has been reviewed by a human. 🏂🏻