### What problem does this PR solve?
The Go chunk pipeline's `PostprocessOperator` `filter` stage
(`internal/ingestion/chunk/postprocess.go`) only filtered by length
(`min_length`/`max_length`). It could not drop empty/whitespace-only
chunks or duplicate chunks — both standard RAG post-processing steps
(blank chunks shouldn't be indexed; identical chunks waste embedding
compute and add redundant retrieval results).
This adds two optional, default-off booleans to the `filter` config:
- `drop_empty` — drop chunks whose content is empty or whitespace-only.
- `drop_duplicates` — drop chunks whose exact content already appeared
(order-preserving; the first occurrence is kept).
They compose with the existing length bounds and are reflected in
`String()` for plan explainability. Also adds the first unit tests for
the postprocess filter (length bounds, drop_empty, drop_duplicates,
combined, exact-content matching, and config parsing).
Validation: `gofmt` clean, `go vet ./internal/ingestion/chunk/` clean,
`go build` ok, `go test ./internal/ingestion/chunk/` — all tests pass.
Closes#16048
### Type of change
- [x] New Feature (non-breaking change which adds functionality)
Co-authored-by: Ling Qin <qinling0210@163.com>
