fix: ignore filesystem events during runtime shutdown

[baszalmstra]

Some Windows runtime tests intermittently abort with 0xc0000409 during teardown. This looks like a stack buffer overrun, but the actual failure is a pending notify callback trying to send a filesystem event after Runtime has dropped its receiver. The resulting panic crosses the non-unwinding Windows callback boundary, so the process aborts.

This ignores that send error. A disconnected receiver means the runtime is shutting down and cannot consume the event anyway.

This PR is stacked on #597.

Testing

• 100 runs of the seven affected mapping and hot-reloading tests
• all 86 mun_runtime tests
• pixi run test-coverage (660 passed, 12 skipped)
• cargo clippy -p mun_runtime --lib -- -D warnings
• cargo fmt --all -- --check