Add coverage for metadata and top-level filter key collisions

[wayyoungboy]

Follow-up from review of PR #1052.

Summary

Add regression coverage that documents the intended behavior when user metadata keys collide with promoted or top-level payload fields used by search filters.

Suggested coverage

• Plain metadata filters versus explicit metadata-prefixed filters for keys such as hash, data, created_at, updated_at, and category.
• SQLite search, list, and count paths.
• OceanBase-like and PGVector-like adapter filter translation paths.

Acceptance criteria

• Tests make the intended filter semantics explicit for colliding keys.
• Existing public filter behavior is either preserved or intentionally documented with updated tests.