fix(nlab-api): skip missing legacy types

[JacobZyy]

Summary

• share one optional generated-source reader across legacy type and enum scans
• skip stale manifest entries whose generated files no longer exist
• preserve contextual failures for all other I/O errors
• cover both symbol readers with one regression test

Verification

• cargo fmt --check
• cargo clippy --locked --all-targets --all-features -- -D warnings
• cargo test --locked