Optimistic send UI + bounded REST timeout for bad networks

[grapeot]

Problem

On a bad network, the Android send button appeared completely dead. Two compounding causes:

1. No optimistic UI — unlike iOS, Android did not insert the user's message into the list until the server round-trip completed, so there was zero visual feedback while the request was in flight.
2. Unbounded REST read timeout — the shared OkHttp client used readTimeout(0) (infinite). A half-open connection could hang for minutes with no error and no UI change.

Fix

• Split the network client (OpenCodeRepository.kt): REST calls now use a bounded 60s read timeout (15s connect / 60s write) so a stuck request fails fast. The SSE stream keeps an infinite read timeout on its own client — it can legitimately be silent for minutes while the agent runs a long tool call, so it must not inherit the bounded REST timeout.
• Optimistic send (MainViewModel.kt, MainViewModelSessionActions.kt): on send, immediately insert an optimistic user row carrying a deterministic msg_<uuid> id, clear the composer, and show a spinner on the send button (ChatInputBar.kt / ChatScreen.kt).
• Reconciliation by id membership: loadMessages / loadMoreMessages merge pending optimistic rows that the server has not echoed yet, and prune ids the server has confirmed. On send failure the row is dropped and the composer text/attachments are restored so the user can retry.
• Server id echo: the client-chosen id is passed through PromptRequest.messageID (the opencode server schema supports messageID, and the server uses it as the user message id), so reconciliation is pure id membership.

Review-driven hardening

Addressed findings from an independent code review:

• Handle the session.error SSE event (MainViewModelSyncActions.kt): prompt_async acknowledges with 204 before the turn runs, so a failure that happens before the user message is persisted (agent missing, session deleted, crash) only surfaces through this event. It was previously dropped, leaving a permanent ghost optimistic row and silently losing the user's text. Now, for the current session, the pending optimistic rows are dropped, their text is recovered into the composer, and an error is set. parseSessionErrorReason builds a bounded display reason (mirrors the iOS handler).
• Session-scoped failure rollback (MainViewModelSessionActions.kt): the send-failure composer restore is now guarded by session ownership, so a failure that lands after the user switched sessions no longer clobbers the other session's draft.
• Host-switch state reset (MainViewModel.kt): resetRuntimeForHostSwitch now clears pendingOptimisticMessageIds so stale ids don't leak across a host switch.

Tests

• Updated all sendMessage mocks/verifies to the new 6-arg signature.
• Optimistic send: optimistic insertion, failure rollback, and success-keeps-row-until-confirmed.
• Cross-session rollback guard (switch sessions mid-send, then fail).
• session.error handling: drop + text recovery, and ignore for a different session.
• Host-switch reset of pendingOptimisticMessageIds.
• OptimisticSendTest.kt covering the pure helpers (makeServerId, buildOptimisticMessage, mergePendingOptimisticMessages, parseSessionErrorReason).
• Full testDebugUnitTest suite is green (327 tests).