Converge `TokenChunker.delimiter_mode` from three values (`token_size`,
`delimiter`, `one`) to two (`delimiter`, `one`). The unified `delimiter`
mode now carries the old `token_size` semantics: when no active
(backtick) delimiter is present, text/JSON chunks are merged up to
`chunk_token_size`; when a backtick delimiter is present, the text is
split by it and not merged. `one` continues to be handled by the
separate `OneChunker`.
## Summary
Follow-up to #17835 (merged). The OVER_CAP / UNDER_CAP merge strategy
was threaded through `mergeDecision` and `mergeByTokenSizeFromJSON` as
an inlined `allowBoundaryOverflow bool` derived from `!c.param.UnderCap`
at three call sites. This replaces that with a named
`schema.MergeStrategy` enum.
## Why
- The `!c.param.UnderCap` inversion was hand-written in three places, so
a future strategy addition could silently drift between the JSON path
(`invokeTextPayload` / `invokeJSONPayload`) and the text path
(`mergeByTokenSize`) — no compile error, and the existing tests don't
cover all three sites with both strategies.
- The strategy concept was never named; `allowBoundaryOverflow` (true =
OVER_CAP) is a double-negation of `UnderCap` and reads opaquely at the
5th positional argument.
## What changed
- Add `schema.MergeStrategy` (`MergeOverCap` / `MergeUnderCap`)
mirroring Python's `rag/nlp/__init__.py` `MergeStrategy`, so Go and
Python stay on the same vocabulary.
- Expose `TokenChunkerParam.MergeStrategy()` derived from the
wire-facing `UnderCap bool` (existing `"under_cap"` configs keep working
— no schema break).
- `mergeDecision` and `mergeByTokenSizeFromJSON` now take
`schema.MergeStrategy` instead of `allowBoundaryOverflow bool`; the
three call sites pass `c.param.MergeStrategy()` (no `!`).
- Tests updated to pass the enum; added a guard test for the `UnderCap`
-> `MergeStrategy` mapping and an end-to-end test for UNDER_CAP on the
JSON path.
No behavior change: default remains OVER_CAP, `under_cap=true` still
selects UNDER_CAP.
## Test plan
`bash build.sh --test ./internal/ingestion/component/chunker/...
./internal/ingestion/component/schema/...` — all green, including
`TestMergeByTokenSizeFromJSON_UnderCapNoOverflow`,
`TestMergeByTokenSize_UnderCapNoOverflow`,
`TestInvokeJSONPayload_UnderCapEndToEnd`, and
`TestTokenChunkerParamMergeStrategy`.
## Related issues
- Relates to #17835 — wired UNDER_CAP as a tested merge-strategy seam
(merged)
- Relates to #17799 — contract doc for token-chunker cap/delimiter
alignment
- Relates to #17808 — related chunker alignment work
---------
Co-authored-by: CodeBuddy <noreply@cnb.cool>
