v2: a declined export's payload is absent in all four bindings; batches.md's unconsumed and payload notes, measured

[EricAndrechek]

What

When chs_preview_batch declines an asked-for export, the v2 library still returns a non-NULL, zero-length out_export buffer. Only the document's export_declined tells declined from emitted-but-empty. The documented contract says a declined export's payload is absent; Go and Python returned an empty payload instead.

• measured on the published v2 dev build 20261011.105019, in Go and Python: a batch rejected by a CHECK, asked to export JSONCompactEachRow, gave the raw buffer len=0 (non-NULL) and a payload of length 0 (Go Payload != nil, Python b""), with export_declined set. An accepted zero-row export gave the same empty payload with export_declined empty, so the two were indistinguishable by payload.
• inferred from the code, not run (TypeScript and Rust have no local build here): both passed the buffer through unchanged as well.

Fix (identical in all four)

The payload and spans are absent (Go nil, Python None, TypeScript undefined, Rust None) whenever the document's export_declined (or export_declined_b64) is non-empty, whatever the buffer. They are present, possibly empty, only for an export that ran. The decision never reads the buffer's length; the library buffer is freed as before.

• Go: decode_batch.go (production path) and decode.go (decodeBatchGeneric); the equivalence reference follows.
• Python: _decode.py. TypeScript: documents.ts. Rust: decode.rs.

Tests

A unit test per binding, fed a declined document with a zero-length (and a non-empty) buffer, expecting absence, and an accepted zero-row export expecting present-and-empty. Go also adds two declined documents to the old-versus-new decoder equivalence set. No stub-level case: the ABI stub's batch documents are fixed and none declines an export with a zero-length buffer, so that needs a new stub variant, left out of this change.

Docs

docs/guides/batches.md: the unconsumed note for a malformed UUID now states the measured answer in place of "unverified", and the payload note states that bindings and raw-ABI callers decide "declined" from export_declined. docs/reference/bindings-v1.md payload row, and each binding's CHANGELOG (### Fixed).

TypeScript and Rust run in CI only.

🤖 Generated with Claude Code