refactor(sandbox): let the runner reset itself for reuse instead of the pool

[ttbombadil]

Purpose

R3b of the refactoring series (docs/UNOSIM_REFACTORING_OPL.md), audit finding A6. Follows R3a (#164).

Finding re-verified

SandboxRunnerPool reset a released runner through a cast to SandboxRunnerInternal and assigned about 20 ExecutionState fields by hand. Any new state field had to be remembered in the pool, or it survived into the next user's run. Part of that code did nothing:

• removeAllListeners on runner, registry and batchers (none are EventEmitters);
• fileBuilder.reset (the method does not exist).

Change

• SandboxRunner.resetForReuse() owns the reset:
  • stops a running run (a failing stop() is logged and the reset continues; before, it aborted the remaining reset);
  • clears the process listeners and the same execution-state fields as before;
  • resets the I/O registry (tolerating failure) and clears the timeout manager.
• The pool only calls runner.resetForReuse() inside its existing reset timeout and stuck-runner replacement. SandboxRunnerInternal, clearRunnerListeners and resetRunnerState are gone (−167 lines).

Tests

• New tests/server/services/sandbox-runner-reset.test.ts against the real runner: state fields, listener clearing, registry reset incl. failure, stop of a running runner incl. failing stop. It was RED before (method missing).
• sandbox-runner-pool.test.ts described the runner's internal reset from the outside; this was the coupling being removed. Its assertions moved, none were dropped:

Pool test before	Now
field resets on release (2 tests), running runner stopped, registry reset called, registry reset failure tolerated	runner tests above; pool test asserts delegation to resetForReuse
reset error tolerated, stuck runner replaced, slot freed while reset hangs	unchanged intent; the mock's resetForReuse rejects or hangs instead of stop

• npm run check, ESLint, unit 2713 passed, runner-reuse race test green, Docker integration 26/26 locally, pre-push incl. Sonar quality gate PASSED.

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