system_interface: guard interrupt-and-send against concurrent restarts

[pseay-imbue]

Problem

In the chat web UI, a queued message exposes an "interrupt and send" action (interruptAndResend in PendingMessageView.ts), which restarts the agent and then re-sends the message. Each queued message bubble renders its own button, and interruptAndResend had no per-agent in-flight guard. With multiple queued messages, triggering the action on more than one fires several interrupt+send sequences concurrently against the same agent. The overlapping restarts repeatedly tear down and re-spawn the agent, so a send from one sequence lands while another's restart is mid-flight, producing:

Failed to interrupt and send to agent : Failed to send message to agent '' (0 successful agents)

MessageInput.ts's handleInterrupt already guards against this with isInterruptInFlight; interruptAndResend was missing the equivalent.

Fix

Add a per-agent in-flight guard to interruptAndResend:

• Module-level const interruptResendInFlight = new Set<string>().
• Return early (no-op) if the agentId is already in the set; otherwise add it and remove it in a finally, so the guard clears on both success and error.
• Existing optimistic-bubble logic (markPendingMessageSending / markPendingMessageQueued / removePendingMessage / failure alert) is unchanged.

Tests

Added three tests to PendingMessageView.test.ts: a held/deferred interruptAgent proves a second concurrent resend for the same agent is a no-op (interruptAgent/sendMessage each called once), plus two asserting the guard clears after success and after failure so a later resend still runs.

Checks (from apps/system_interface/frontend): npm test 153 passed / 0 failed; npm run lint clean; npm run format:check clean.

Scope / follow-ups

• This is the web UI half of a two-part fix. The deeper fix to the mngr send path (waiting for the agent's TUI to be ready before sending) is being made separately against the mngr repo.
• Out of scope: "resend all queued messages after one interrupt" (a restart clears the agent's queue, so only the resent message survives) remains a possible future enhancement.

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