Fix ruststreamer EOS recording finalization regression

Investigate and fix the deployed ruststreamer shutdown regression where pipeline EOS times out and final bookmark metadata is attempted against an unfinalized MP4 segment. Acceptance criteria: - Normal stream shutdown finalizes the active splitmuxsink fragment before metadata duration probing. - Teardown does not wait for an unreachable pipeline-level EOS condition. - Recording and bookmark finalization failures remain visible and continue to prevent unsafe processing. - Regression coverage exercises the shutdown event ordering without requiring production RTMP/S3. - Rust formatting, clippy, checks, and relevant tests pass.