fix(mothership): close a pre-aborted SSE stream, keep the new-chat effort across a failed first send - #8754

Summary

Three findings from the #8748 release review.

• A pre-aborted SSE request no longer subscribes. createSSEStream adds its abort listener inside start(), and that listener never fires for a signal that's already aborted. A client that left while the route was still authorizing got a stream that subscribed anyway and held its handler, timers and a revalidate query every 30 s until rotation (~16.5 min). start() now closes at once with aborted before taking any subscription. Covers all three callers: the desktop inbox stream, mothership events and createWorkspaceSSE.
• The new-chat effort survives a failed first send. ModelSelector cleared the pick on unmount whenever it had no chat id. A first send swaps the empty-state composer for the chat view, so the pick was wiped mid-send: the chat view showed the default while pending, and once rollbackOptimisticSend restored the pick, the swapped-back composer's unmount wiped it again. The pick belongs to the chatless surface, not one composer, so useChat now owns its lifetime: it drops when the surface unmounts, switches chats, or adopts a chat (adoptResolvedChatId, so a first send stopped before admission doesn't carry it into the next new chat). Admission still moves it onto the new chat via adoptNewChatEffort first. The rollbackOptimisticSend restore is gone: nothing clears the pick while a send is pending, and it overwrote a pick made in the chat view during the send. Covers every useChat surface: Home, organization home and the workflow copilot panel.
• effort is optional on the chat response. It was required, so during a deploy's traffic shift a new client hitting an old server failed contract validation and the chat didn't load. parseChatHistory already maps a missing value to null; MothershipChatHistory is unchanged.

Type of Change

• Bug fix

Testing

Each new test fails on staging:

• never subscribes when the request aborted before the stream started (sse-endpoint.test.ts): expected false to be true, the body never finishes.
• keeps the new-chat effort across the composer swap of a first send that fails (use-chat.dom.test.tsx, renders the real ModelSelector in a Home-shaped swap): expected 'High' to be 'Low' while pending, and with that line removed, expected null to be 'low' after the failure.
• keeps a new-chat effort picked while the first send is pending when that send fails: expected 'Low' to be 'Medium' with the rollback restore.
• starts the next new chat at the default after a first send stopped before admission: expected 'Low' to be 'High' without the adoption clear.
• drops an unsent new-chat effort when the surface leaves the page / opens another chat pins the remaining abandonment paths on the new layer.
• loads a chat from a server that predates the effort field (mothership-chats.test.ts): Response failed contract validation — chat.effort.

apps/sim typecheck, biome, check:api-validation, check:api-contract-routes, check:openapi, check:comment-hygiene, check:test-patterns and the home, hooks/queries, contracts and sse-endpoint suites (1031 tests) pass.

Checklist

• Code follows project style guidelines
• Self-reviewed my changes
• Tests added/updated and passing
• No new warnings introduced
• I confirm that I have read and agree to the terms outlined in the Contributor License Agreement (CLA)