fix: preserve task stop channel during close

[besnow]

Motivation

• Fix a race in Task.Close() where close(t.stop) followed by t.stop = nil could make the worker goroutine observe a nil t.stop after an Execute() return and lose the stop signal.
• Preserve the existing duplicate-close safety while ensuring the worker listens on the original stop channel created in Start().

Description

• Capture the channel created in Start() into a local variable with stop := t.stop immediately after t.stop = make(chan struct{}).
• Change the worker goroutine select to listen on the local stop instead of t.stop (case <-stop:).
• On Execute() error paths, under t.access lock, only close(stop) and set t.stop = nil when t.running && t.stop == stop to avoid double-close races.
• Keep the existing Close() implementation unchanged.

Testing

• Ran gofmt -w common/task/task.go which formatted the file successfully.
• Ran go test ./... and the test suite completed with all tests passing.
• Ran go build ./... and the project built successfully.

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Codex Task