gcvctor: JSONFromNullable/PGJSONBFromNullable string special-case diverges from the client; prefer json.RawMessage for pre-encoded wire

[apstndb]

Feedback from spanenc adoption of v0.7.2 (#232 / #207).

Problem

JSONFromNullable and PGJSONBFromNullable store a string Value as the wire JSON as-is. The official client's encodeValue (mirrored by spanenc) always json.Marshals NullJSON.Value / PGJsonB.Value, so a Go string becomes a quoted JSON string on the wire:

Input	client wire	gcvctor v0.7.2 wire
NullJSON{Value: "x", Valid: true}	"x"	x (invalid JSON)
NullJSON{Value: "{\"a\":1}" /* Go string */, Valid: true}	"{\"a\":1}"	{"a":1}

This made the helpers unusable for spanenc (which kept its own marshal-always encodeNullJSON / encodePGJsonB, see apstndb/spanenc@662f7a8), and it is a trap for anyone coming from the client: the wire meaning of Value: "x" silently changes with the dynamic type.

Note that a client-decoded JSON string column yields a Go string in NullJSON.Value; re-encoding through these helpers corrupts that round trip ("x" becomes x).

Proposal

Drop the string special-case and always delegate to JSONValue / PGJSONBValue. The double-marshal-avoidance use case is served by the established Go idiom for pre-encoded JSON, json.RawMessage, which jsonWireString's json.Encoder already passes through (validated and compacted, no HTML escaping) — and which the client itself also passes through, keeping the two aligned.

• Value is a string: quoted JSON string (client-compatible)
• Value is a json.RawMessage: wire as-is (the explicit pre-encoded path; client-compatible modulo the already-documented compact/HTML-escape cosmetic divergence)

With this change spanenc can replace its hand-rolled encodeNullJSON / encodePGJsonB with these helpers.

Impact

Runtime behavior change for string Values relative to v0.7.2 (released 2026-06-11; the special case shipped in #232 and has no known adopters — spanenc explicitly declined it). Release versioning is the maintainer's call; precedent: v0.7.1 shipped wire-correctness fixes as a patch.

[apstndb]

Reframing after discussion: this should be treated as a bug fix, patch-eligible (v0.7.3), not a design change.

The *FromNullable helpers take the client's own wrapper types, whose Value semantics the client defines (a decoded JSON string column yields a Go string). Interpreting client-typed input differently from the client is a contract violation, and the v0.7.2 behavior emits invalid wire (NullJSON{Value: "x"} produces bare x). This is the same class as the v0.7.1 wire-correctness fixes (TimestampValue UTC normalization, JSON HTML-escape removal), which shipped as a patch. PR #237 updated accordingly.