gcvctor: marshal string Values in JSONFromNullable/PGJSONBFromNullable like the client

[apstndb]

Closes #236. Feedback from the spanenc adoption of v0.7.2 (#232 / #207).

What

JSONFromNullable and PGJSONBFromNullable no longer special-case a string Value as pre-encoded wire JSON. They always delegate to JSONValue / PGJSONBValue, so a Go string marshals to a quoted JSON string on the wire — the same semantics as the official client's encodeValue. Pre-encoded wire JSON is passed as json.RawMessage, which jsonWireString's json.Encoder already stores as-is (validated and compacted, no HTML escaping); the client follows the same json.Marshal convention for RawMessage, keeping the two aligned.

Input Value	v0.7.2 wire	this PR wire	client wire
"x" (string)	x (invalid JSON)	"x"	"x"
json.RawMessage("{\"a\":1}")	{"a":1} (via marshal)	{"a":1}	{"a":1}

Why this is a bug fix, not a design change

These helpers take the client's own wrapper types, whose Value semantics the client defines: a client-decoded JSON string column yields a Go string in NullJSON.Value, and re-encoding it through the v0.7.2 helpers corrupts the round trip ("x" becomes bare x, which is not valid JSON wire). Interpreting client-typed input differently from the client is a contract violation, and the output is malformed — the same class as the v0.7.1 wire-correctness fixes (TimestampValue UTC normalization, JSON HTML-escape removal), which shipped as a patch.

The divergence is also what kept spanenc on its hand-rolled encodeNullJSON / encodePGJsonB (apstndb/spanenc@662f7a8). With this fix spanenc can adopt these helpers. json.RawMessage is the established encoding/json idiom for pre-encoded payloads, so the double-marshal-avoidance use case keeps an explicit, client-compatible spelling.

Changes

• gcvctor/nullable_input.go: drop the string special-case in both helpers; document the marshal semantics and the RawMessage path.
• gcvctor/doc.go: note the behavior in the nullable-inputs section.
• gcvctor/example_test.go: ExampleJSONFromNullable pinning string-vs-RawMessage wire output.
• gcvctor/nullable_input_test.go: update string-Value expectations to quoted wire; add RawMessage pass-through and invalid-RawMessage error cases.

Release note

Recommend shipping as a patch (v0.7.3): it fixes invalid wire output from a helper introduced in v0.7.2 (released 2026-06-11) with no known adopters — spanenc, the requester of #232, explicitly declined the special case. Suggested release-notes row:

Area	v0.7.2	v0.7.3
JSONFromNullable / PGJSONBFromNullable with string Value	Wire as-is (invalid for non-JSON strings)	Quoted JSON string (client-compatible); use json.RawMessage for as-is

🤖 Generated with Claude Code

[gemini-code-assist]

Code Review

This pull request updates JSONFromNullable and PGJSONBFromNullable to marshal Go strings as quoted JSON strings, aligning with the official Spanner client's encodeValue behavior. To store pre-encoded wire JSON as-is, users must now pass it as json.RawMessage. The PR also updates the documentation, adds examples, and adjusts the unit tests to reflect and verify this behavior. I have no feedback to provide as the changes are clean, well-documented, and properly tested.

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