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Jack 310275208b refactor(chunker): replace allowBoundaryOverflow bool with MergeStrategy enum (#17851)
## 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>
2026-08-05 14:59:53 +08:00
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