translate: read by-value DXL datums without over-reading the payload

[yjhjstz]

TranslateDXLDatumGenericToScalar() copied sizeof(Datum) bytes out of a CDXLDatumGeneric payload that only holds typlen bytes, so every by-value type narrower than a Datum -- date, float4, "char", xid, cid, the reg* types and user enums -- picked up memory past the end of that buffer and dropped it into the high half of Const->constvalue. With the palloc-backed gpos pool the read lands in the alignment padding of the surrounding chunk, so under an assert build the constant for DATE '2026-03-01' came out as 0x7f7f7f7f00002554 (the 0x7f bytes being CLOBBER_FREED_MEMORY fill); "char", at one payload byte, drags in seven foreign bytes.

Read exactly the payload with fetch_att() instead. Besides bounding the read it widens the value the way PostgreSQL does when it builds such a Const itself: Int32GetDatum() and friends sign-extend, so DATE '1990-01-01' yields 0xfffffffffffff1bc, matching date_in(). A zero-extending copy of constlen bytes would also stop the over-read but produce a non-canonical Datum that no longer compares equal to a PG-built constant of the same value under datumIsEqual(), which is what _equalConst() uses for by-value types. The payload length is validated against typlen up front, so a DXL datum that disagrees with the metadata raises a conversion error and falls back rather than reading out of bounds in a release build.

The TVF-evaluates-to-const path in CTranslatorDXLToPlStmt carried its own copy of this code that additionally assumed every datum is generic and passed by reference, leaving constbyval unset for by-value types and mis-casting int4, bool and oid datums. Route it through the scalar translator, as every other Const site in that file already does.

Found via warehouse-pg/warehouse-pg#114, which fixes the same over-read with a zero-extending memcpy in a CTranslatorUtils helper; this takes a different approach, so expect a conflict when that commit is merged.

[cr-gpt]

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