fix(runtime): unblock canceled MicroSandbox input streams

[skevetter]

Canceled MicroSandbox commands could remain blocked while copying an owned input stream, even after the child process exited. Agent binary transfer had the same problem because its progress wrapper hid the source closer. Close eligible non-file input on cancellation, join the callback before returning, and close each acquired transfer source exactly once so retries receive a fresh source. Normal completion, failed starts, caller-owned files, existing transfer limits, and original cancellation causes retain their behavior.

Add six actual-VM scenarios across the built-in driver and published external provider v0.1.7: completed stdout/stderr logs while running and stopped, cancellation of a ready parent/child tree, and cancellation with blocked owned stdin. Independent PID/start-time/nonce observation checks process death within the same ten-second cancellation bound; VM creation time, boot ID, subsequent command execution, and source contents must survive. The external test path uses the real self-executed runtime supervisor and prepared executable. Each cleanup owns only its recorded process or previously absent VM identity.

Validation:

• Old production fails the new blocked-input and transfer-idle regressions; fixed focused race tests pass, including ownership, joining, fresh retries and original causes.
• Final affected-package uncached race, vet, strict lint, all 13 pre-commit hooks, module verification, formatting and diff checks passed. The six-case Ginkgo dry run only verifies selection; native proof requires CI.
• Integrated the completed Dockerless slice. Fresh full committed local CodeRabbit reviewed all eight PR files with zero findings. Current-head CI, including all existing VM suites and the six new logs/cancellation specs, Greptile and full remote CodeRabbit remain pending.

This does not establish TTY or arbitrary nonclosable-input cancellation, binary log fidelity or ordering, runtime compatibility failures, invalid-recreate preservation, host hardening, or existing-workspace migration. Built-in-provider cutover remains separately gated.

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