Large binary accepts incomplete S3 references

### Feature Summary

The `largebinary` constructor only checks the `s3://` prefix. References with no bucket or no object key are accepted and fail later when an input or output stream calls S3.

Expected behavior: malformed references fail immediately with a clear validation error.

Before: `s3://`, `s3:///object`, and `s3://bucket` are accepted

After: a reference must contain both a bucket and object key

Reproduction evidence:

Construct `largebinary("s3://")` and inspect its bucket and object key. Both are empty although construction succeeds.

Version and commit evidence:

1.3.0-incubating-SNAPSHOT on main

**Commit Hash (Optional)**
50321e403c82df299a13deb50a7f9849dd93bdba

**Browsers**

Not browser specific

**Relevant log output**

Malformed references are accepted and later reach S3 with an empty bucket or key.

### Proposed Solution or Design

Expected behavior: malformed references fail immediately with a clear validation error.

After: a reference must contain both a bucket and object key

### Affected Area

Workflow Engine (Amber)