fix(core): allow agents to stop background shell commands

[dbpolito]

Issue for this PR

Closes #36424

Type of change

• Bug fix
• New feature
• Refactor / code improvement
• Documentation

What does this PR do?

Adds shell_stop so agents can cancel a background command using its returned shell ID. It reuses Job.cancel and the existing interruption cleanup (shell.remove), including removal of captured output.

Related: #46721 adds stop APIs and user-stop outcomes; this PR adds the model-callable tool using current cancellation behavior.

How did you verify your code works?

• All 35 pre-push typecheck tasks passed.
• Both integration tests passed: tool cancellation and interruption cleanup, including removal and cancellation notifications.
• The earlier full shell test run had six Bash process-substitution/compound-syntax failures.

Screenshots / recordings

Not applicable (no UI changes).

Checklist

• I have tested my changes locally
• I have not included unrelated changes in this PR

[github-actions]

Thanks for your contribution!

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Please:

1. Open an issue describing the bug/feature (if one doesn't exist)
2. Add Fixes #<number> or Closes #<number> to this PR description

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