fix(ctx_execute): prevent hang when background=true without timeout

[ayuayue]

Problem

When ctx_execute is called with background: true but no explicit timeout, the tool call hangs forever and the session becomes stuck.

Root cause

Two conflicting code paths:

1. server.ts appends a permanent setInterval(()=>{}, 2147483647) to keep the backgrounded process alive
2. executor.ts only creates the timer that triggers detach when timeout is explicitly set

With background: true and no timeout:

• The infinite interval keeps the Node process alive forever
• No timer fires to trigger the detach logic
• The process never exits → "close" event never fires → res() never called → tool call hangs

Fix

Only append the keepalive interval when a timeout is actually configured (effTimeout !== undefined). Without a timeout, the process exits naturally when its code finishes, the "close" event fires, and the tool call completes normally.

Changes

• src/server.ts: Move effTimeout = resolveExecTimeout(timeout) before the instrumented code template, and gate the infinite interval on background && effTimeout !== undefined

Impact

Scenario	Before	After
background: true + timeout	✅ Normal (detach on timeout)	✅ Unchanged
background: true + no timeout	❌ Tool call hangs forever	✅ Process exits naturally, tool call completes
No background flag	✅ Normal	✅ Unchanged