fix(core): clarify user-directed tool cancellation

[AaronZ345]

What this PR does

• Updates the model-facing message for post-completion tool cancellations to make the user's Esc action explicit.
• Keeps the existing discarded-output signal while telling the model to stop and await further instructions instead of retrying or working around the cancelled tool call.
• Adds a regression assertion for the post-completion cancellation branch.

Why it's needed

When a user presses Esc while a tool is running, the cancellation can arrive after the tool has already produced output. The previous message only said the tool had already completed and its output was discarded, which can be misread as a transient tool/runtime issue and cause the model to retry. This makes the user intent clear.

Reviewer Test Plan

• npm -w @qwen-code/qwen-code-core test -- src/core/coreToolScheduler.test.ts -t "post-completion cancellation"
• npm -w @qwen-code/qwen-code-core test -- src/core/coreToolScheduler.test.ts
• npm -w @qwen-code/qwen-code-core run lint

Risk & Scope

• Low risk: only changes the cancellation text returned to the model for the post-completion cancellation branch.
• No scheduler state-machine or tool execution behavior changes.

Linked Issues

Fixes #10170