## Summary
Migrate the metadata filtering utilities `meta_filter` and
`convert_conditions` from `common/metadata_utils.py` to Go as pure
functions with zero external dependencies.
These functions are used by `dify/retrieval`, `openai/chat/completions`,
`document_api`, and `chunk_api` for filtering documents by metadata
conditions.
### Changes
- **New**: `internal/common/metadata_utils.go` — `ConvertConditions()`
and `MetaFilter()` with full operator support
- **New**: `internal/common/metadata_utils_test.go` — 18 test cases
covering all operators and edge cases
### Supported Operators
`=`, `≠`, `>`, `<`, `≥`, `≤`, `contains`, `not contains`, `in`, `not
in`, `start with`, `end with`, `empty`, `not empty`
### Design
- Numeric comparison via `strconv.ParseFloat`
- Date comparison via YYYY-MM-DD format detection
- Case-insensitive string comparison fallback
- `and` / `or` logic support for multiple conditions
- Zero external dependencies — pure functions only
