fix(ostool-server): clear serial queue after session

[ZR233]

Summary

• Flush the serial stream and clear all tty buffers before closing an ostool-server serial websocket session.
• Preserve the original websocket/session result when cleanup fails, logging cleanup failures as warnings instead.
• Add serial cleanup unit tests for flush/clear ordering, cleanup error reporting, and preserving the original session error.

Validation

• cargo fmt --all -- --check
• cargo clippy --target x86_64-unknown-linux-gnu -p ostool-server --all-features
• cargo test -p ostool-server