test(decode): collapse runtime success paths in boundary

[seasonyuu]

What changed

• collapse decode runtime success-path assertions in crates/dsview-sys/tests/boundary.rs into a single runtime lifecycle
• keep the missing-decoder-dir error check separate
• avoid repeated successful init -> exit -> init cycles in one boundary test binary

Why

Linux CI showed that dsview-sys boundary tests could corrupt heap state when decode runtime metadata success-path tests repeatedly initialized and exited the decode runtime in the same process.

This change keeps the same contract coverage while matching the runtime usage pattern more closely and avoiding the failing repeated-successful-init path.

Validation

• local: cargo test -p dsview-sys --target x86_64-unknown-linux-gnu --test boundary -- --nocapture --test-threads=1
• local with diagnostics: MALLOC_CHECK_=3 PYTHONMALLOC=debug RUST_BACKTRACE=full DSVIEW_DECODE_DEBUG=1 cargo test -p dsview-sys --target x86_64-unknown-linux-gnu --test boundary -- --nocapture --test-threads=1
• CI run where Linux passed: https://github.com/LISTENAI/dsview-cli/actions/runs/24773735361