feat(json): allow @JsonWriter on enum instance method and property getter

[dsudomoin]

Motivation

Enum @JsonWriter (from #701) required the value extractor to be a static method — a Kotlin companion object function (or Java public static) taking the enum and returning the JSON value. Putting @JsonWriter on a plain instance method produced a hard must be static error, and there was no property-level form. This PR broadens the accepted shapes so the common cases read naturally.

What changed

@JsonWriter on an enum now additionally accepts:

• instance method — @JsonWriter fun jsonValue(): V (0 params), including @JsonWriter override fun toString(): V;
• Kotlin property getter — @get:JsonWriter val value: V (KSP only; Java has no property concept).

The static (companion) form is unchanged and validated exactly as before. The forms are disambiguated by parameter count:

form	shape
static (companion / public static)	exactly 1 param of the enum type
instance method	0 params
Kotlin property getter	@get: on the property

Value-source priority is unchanged: @JsonWriter (any form) → @Json getter → toString.

Examples

enum class AssetType(val value: String) {
    CURRENCY("currency"), OTHER("other");

    // any one of these now works:
    @JsonWriter fun toValue(): String = value               // instance method
    // @JsonWriter override fun toString(): String = value   // toString override
    // companion { @JsonWriter fun toValue(t: AssetType) = t.value }  // static (as before)
}

// property getter form:
enum class AssetType2(@get:JsonWriter val value: String) { A("a"), B("b") }

Note: a bare @JsonWriter val value (no use-site target) is still rejected by the Kotlin compiler, because @JsonWriter is @Target({TYPE, METHOD}) — use @get:JsonWriter.

Implementation

No runtime change — the generated writer still delegates to EnumJsonWriter<T,V>; only the extractor code block and the value type change.

• KSP EnumJsonWriterGenerator.detectWriterMethod: accepts body methods and getter-annotated properties in addition to companion methods. An instance member is emitted as Enum::member (a (Enum) -> V), the static one as { e -> Enum.m(e) }.
• KSP JsonSymbolProcessor: triggers writer generation for a @get:JsonWriter property, handling both KSPropertyDeclaration and KSPropertyGetter (the symbol reported by getSymbolsWithAnnotation for a @get: target varies by KSP version).
• APT EnumWriterGenerator.detectWriterMethod: accepts instance methods; the existing Enum::method codegen already resolves to Function<Enum,V> for both static and instance forms, so generateEnumWriter is untouched. (Property getters are Kotlin-only, so APT stays method-only.)

Tests

Added to EnumTest (KSP + APT), covering: instance method, toString override, property getter (with and without @Json, to exercise the generation trigger), the from-extension path, and negative cases (instance method with params → no parameters; non-public → must be public). Full :json:json-symbol-processor:test and :json:json-annotation-processor:test are green.