fix(native-sidecar): close WASM pipe writers on child exit

[atirna]

Summary

• Release a WASM child's inherited pipe descriptors when its exit is observed.
• Keep the exit event available to child_process.poll for the parent's later waitpid handling.

Fixes #1959

Why

Pipelines can now observe EOF after a writer exits instead of waiting for the blocking-read watchdog.

Verification

• cargo test -p agentos-native-sidecar wasm_child_exit_closes_kernel_writer_before_parent_poll --lib
• cargo fmt --check -- crates/native-sidecar/src/execution/child_process.rs