## Summary
This PR fixes case-sensitivity regressions introduced in #15656 and
consolidates the metadata filtering pipeline by removing the duplicate
`applySingleCondition` adapter layer.
### Bug fixes
1. **contains / not contains**: restored case-insensitive matching (was
lost when `applySingleCondition` was replaced by
`common.MetaFilter.matchValue` which lacked `strings.ToLower`)
2. **not in**: restored case-insensitive matching (was lost for same
reason; uses `strings.EqualFold`)
3. **!= with date filter values**: non-date metadata values now
correctly match the `≠` operator (a non-date value IS not equal to any
date, but was returning false)
### Architecture
4. **Removed `applySingleCondition`** (65 lines) — the inline switch was
a duplicate of `common.MetaFilter` logic. `ApplyMetaFilter` now converts
conditions and delegates to `common.MetaFilter` once per filter set,
eliminating ~25 lines of duplicate AND/OR merge logic.
5. **Added `filterSet`** — O(n+m) hash-map fast path for `in`/`not in`
operators, replacing the O(n*m) linear scan in `matchValue`.
6. **Exported `NormalizeOperator`** from `common` for consistent
operator alias handling.
### Cleanup
7. Removed 18 lines of dead code (`matchValue`'s `in`/`not in` branches
already bypassed by `filterOut` delegation)
8. Fixed orphaned godoc comment for `convertOperator`
9. Fixed incorrect `filterSet` doc comment (claimed "matching EqualFold"
but used `strings.ToLower`)
10. Completed `convertToMetaCondition` operator normalization
documentation
### Testing
- 60 tests (24 service + 36 common), all passing
- New tests: `==`, `≠`, `>`, `<`, `≥`, `≤`, `empty`, `not empty` through
`ApplyMetaFilter`
- New tests: `<`, `≤`, `≠` through `MetaFilter`; `not-in-empty-list`
through `filterSet`
- All 18 `MetaFilter` tests pass; all 10 `filterSet` unit tests pass
---------
Co-authored-by: Claude Opus 4.8 <noreply@anthropic.com>
## 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
