fix: reject ragged multi-vector embeddings

[pragnyanramtha]

Summary

• validate multi-vector embeddings before gRPC byte packing
• reject empty/ragged multi-vectors with WeaviateInvalidInputError instead of silently producing malformed bytes
• add byte packing and query construction regression coverage

Validation

• .venv/bin/pytest test/collection/test_byteops.py -q
• .venv/bin/pytest test/test_get_vector_v4.py test/collection/test_queries.py -q
• .venv/bin/ruff format --check weaviate/collections/grpc/shared.py test/collection/test_byteops.py
• .venv/bin/flake8 weaviate/collections/grpc/shared.py test/collection/test_byteops.py
• git diff --check

[orca-security-eu]

Orca Security Scan Summary

Status	Check	Issues by priority
Passed	Infrastructure as Code	0   0   0   0	View in Orca
Passed	SAST	0   0   0   0	View in Orca
Passed	Secrets	0   0   0   0	View in Orca
Passed	Vulnerabilities	0   0   0   0	View in Orca

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