Fix Duplicate task execution after network reconnection

[spider-yamet]

Description

Fixes duplicate task execution when the SSE connection is lost and then restored (e.g. network reconnect, Wi‑Fi disconnect/reconnect, or system sleep/wake). Previously, fetch-event-source would retry on connection errors even when the task had already finished, which could re-run the same task.

What changed

• In chatStore.ts, the SSE onerror handler now checks if the task status is FINISHED before allowing a retry. If the task is already finished, it logs that retry is being stopped to avoid duplicate execution, cleans up the connection, and throws so fetch-event-source does not retry.
• Added a unit test in chatStore.test.ts: "SSE onerror - no retry when task already finished (issue [Feature Request] Duplicate task execution after network reconnection and system wake-up #1212)" to ensure fetchEventSource is only called once and the "already finished, stopping retry" path is exercised when the task is FINISHED and an error occurs.

Verification

• Run: npm run test -- --run test/unit/store/chatStore.test.ts -t "issue #1212"

Closes #1212

What is the purpose of this pull request?

• Bug fix
• New Feature
• Documentation update
• Other

[bytecii]

Thanks, in general LGTM. We can merge if @4pmtong thinks it's good to be merged

[spider-yamet]

@Wendong-Fan @4pmtong Could you please review my PR?