fix: fall back to prefix filters for volcengine path scope

[haosenwang1018]

Summary

• compile PathScope as a prefix filter in the Volcengine adapter instead of emitting the generic must + para=-d DSL
• keep the generic PathScope compilation unchanged for other backends
• add a focused regression test for Volcengine adapter filter compilation

Why

Volcengine backend users reported that target_uri scoped search returns zero results because the cloud backend does not honor the generic para=-d path filter form. This change uses the backend's existing prefix filter support for URI scoping.

Testing

• ruff check openviking/storage/vectordb_adapters/volcengine_adapter.py tests/storage/test_volcengine_clients.py
• ruff format --check openviking/storage/vectordb_adapters/volcengine_adapter.py tests/storage/test_volcengine_clients.py
• git diff --check

Notes

• Full local pytest validation was blocked by repository-specific AGFS build requirements and optional dependency imports during test bootstrap, so this PR keeps scope tight and locks behavior with a focused adapter regression test.

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