[codex] fix batch reference serialization mutation

[pragnyanramtha]

Summary

• avoid mutating BatchReference.to_object_collection while converting references to the internal REST payload
• add coverage that repeated _to_internal() calls preserve the model and produce stable beacons for single-target and multi-target references

Why

BatchReference._to_internal() appended / directly to to_object_collection. Reusing or reserializing the same reference could therefore change Target to Target/ and then emit malformed beacons such as Target//<uuid>.

Validation

• .venv312/bin/python -m pytest test/collection/test_batch.py -q
• .venv312/bin/ruff format --check weaviate/collections/classes/batch.py test/collection/test_batch.py
• .venv312/bin/ruff check weaviate/collections/classes/batch.py test/collection/test_batch.py

[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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