fix(chat): use paginatedVisibleMessages for empty/skeleton routing to fix blank chat on load

[devin-ai-integration]

Summary

Fixes a bug where loading the app sometimes shows a blank chat area (no messages, no skeleton, no greeting) despite a conversation being selected in the sidebar. ChatView's display routing (isEmptyState, shouldShowSkeleton) checked viewModel.messages.isEmpty, but the actual rendering used viewModel.paginatedVisibleMessages — these come from different @Observable objects (ChatMessageManager vs ChatPaginationState) connected via Combine, and can desync when the _modify accessor defers its Combine publish or when all messages are filtered by ChatVisibleMessageFilter. The fix uses paginatedVisibleMessages.isEmpty for both routing checks so branching is consistent with what's actually displayed.

Also removes the unnecessary Combine re-subscription with dropFirst() in resetMessagePagination() — the ChatPaginationState is per-ChatViewModel so the existing subscription from init already tracks the correct messageManager, and the method already calls recomputeVisibleMessages synchronously.

Review & Testing Checklist for Human

• Build in Xcode — CI skips macOS builds; this PR cannot be verified without a local Xcode build
• Relaunch test — Quit and relaunch the app 5+ times; verify conversations always load messages (or show skeleton → messages) instead of a blank chat area
• Conversation switching — Switch between multiple conversations rapidly and verify the correct state always appears (skeleton → messages, or empty state greeting for new conversations)
• All-filtered edge case — If a conversation has only hidden/subagent messages, the view will now show the empty state greeting instead of a blank screen; confirm this is acceptable UX

Notes

• Root cause was identified through static analysis — the intermittent nature makes on-demand reproduction difficult. The fix makes the system robust against any desync between messages and paginatedVisibleMessages regardless of specific trigger.
• resetMessagePagination() is never called externally in the current codebase, so removing its re-subscription is low-risk. The original subscription from init remains active.
• iOS ChatContentView uses a different routing pattern and is not affected.

Link to Devin session: https://app.devin.ai/sessions/ca7df320069f4c0da87d33cfa3e401ab
Requested by: @Jasonnnz

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[devin-ai-integration]

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[devin-ai-integration]

Devin Review found 1 potential issue.

View 2 additional findings in Devin Review.

[devin-ai-integration]

🚩 Subtle behavior change: empty state now shown when messages exist but are all hidden

The switch from viewModel.messages.isEmpty to viewModel.paginatedVisibleMessages.isEmpty at clients/macos/vellum-assistant/Features/Chat/ChatView.swift:106 and :110 introduces a semantic change. If a conversation has messages but they are all hidden (subagent notifications, hidden messages, or messages without renderable content filtered by ChatVisibleMessageFilter), paginatedVisibleMessages will be empty while messages would not be.

Previously, this would render activeConversationContent (empty message list + composer). Now it renders the empty state view (greeting + built-in composer). Both paths provide a working composer, so the user can still interact. This is likely an intentional UX improvement — showing the empty state greeting when nothing is visible is arguably better than an empty message list. However, the reviewer should confirm this edge case (all-hidden messages) was considered.

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[devin-ai-integration]

This is an intentional improvement, already documented in the PR description under the "All-filtered edge case" checklist item. When all messages are filtered (hidden/subagent/phantom), showing the empty state greeting with a working composer is better UX than showing a blank message list with a composer — the user gets context that the conversation is empty from their perspective rather than staring at a blank screen.