gcvctor: StringBasedValueOf(typ, string) — string-payload constructor for arbitrary Types (parallel to BytesBasedValueOf)

[apstndb]

Problem

gcvctor has an asymmetry between bytes-backed and string-backed wire payloads:

• Bytes: BytesBasedValueOf(typ *sppb.Type, v []byte) accepts an arbitrary Type, which is what ProtoValue builds on.
• Strings: only StringBasedValueFromCode(code sppb.TypeCode, v string), limited to simple scalar type codes (and gcvctor: doc clarifications for known traps (BytesValue vs BytesFromSlice, ArrayValue spread, StringBasedValueFromCode scope) #222 already tracks documenting that ARRAY/STRUCT codes produce malformed Types).

This leaves no constructor for string-payload values whose Type is not a bare code — most concretely PG-dialect annotated scalars. spanner.PGNumeric carries a raw wire string (not *big.Rat), so spanenc has to hand-assemble the GCV literal today:

// spanenc encode.go — the only hand-built GCV in the package
return gcv{
        Type:  typector.PGNumeric(),
        Value: structpb.NewStringValue(v.Numeric),
}

Suggested fix

Add the parallel constructor:

// StringBasedValueOf constructs a GenericColumnValue with an arbitrary
// string-compatible Type and the wire string stored as-is (no validation),
// like BytesBasedValueOf for base64 payloads.
func StringBasedValueOf(typ *sppb.Type, v string) spanner.GenericColumnValue

with the same no-validation caveats as StringBasedValueFromCode (caller supplies the canonical wire form). StringBasedValueFromCode then reads as the simple-code convenience over it.

This also unblocks the spanner.PGNumeric counterpart discussed in #207 (its FromNullable needs a raw-string path, not PGNumericValue(*big.Rat)).

Related

• gcvctor: constructor coverage gaps (FromNullable NUMERIC/JSON/INTERVAL, Must* variants, PG_OID, UUID/JSON string inputs) #207 (constructor coverage gaps; PG wrapper decision)
• gcvctor: doc clarifications for known traps (BytesValue vs BytesFromSlice, ArrayValue spread, StringBasedValueFromCode scope) #222 (StringBasedValueFromCode scope documentation)

Feedback from building spanenc's encodeValue mirror on gcvctor.

[apstndb]

Priority raised (spanenc downstream). spanenc is blocked on this for `encodePGNumeric` — the only hand-assembled GCV in that package. Implementation PR incoming on `feat/spanenc-gcvctor-constructors`.

[apstndb]

Landed via #234 (squash-merged). spanenc can now adopt StringBasedValueOf, PGNumericFromNullable, PGJSONBFromNullable, JSONFromNullable, and the other nullable constructors instead of hand-rolled encode helpers.