Feature: opt-in per-constant descriptions for enum in JacksonOutputConverter

[sumitvairagar]

Closes #2028

What

Adds JacksonOutputConverterOption.ENUM_CONSTANT_DESCRIPTIONS — an opt-in flag that installs a CustomDefinitionProviderV2 to emit per-constant @JsonPropertyDescription annotations as oneOf/const/description instead of a bare enum array.

val converter = JacksonOutputConverter(
    MyResult::class.java,
    objectMapper,
    options = setOf(JacksonOutputConverterOption.ENUM_CONSTANT_DESCRIPTIONS),
)

For an enum like:

enum class Priority {
    @JsonPropertyDescription("Needs same-day response") URGENT,
    @JsonPropertyDescription("Standard turnaround")    NORMAL,
    OTHER
}

the schema becomes:

{ "oneOf": [
    { "const": "URGENT", "description": "Needs same-day response" },
    { "const": "NORMAL", "description": "Standard turnaround" },
    { "const": "OTHER" }
]}

Why opt-in

As discussed in #2028, oneOf is not supported by OpenAI native structured-output strict mode and causes schema fallback to the prompt-based path on that provider. The opt-in flag makes this a conscious choice — consumers on Google GenAI, Anthropic, or the prompt-based path can enable it; the default path stays unchanged and OpenAI strict mode is unaffected.

Enums whose constants carry no @JsonPropertyDescription are unaffected regardless of the option.

Tests

Four new tests in EnumConstantDescriptionTests:

• without option: bare enum array (unchanged behaviour)
• with option: annotated enum becomes oneOf/const/description
• with option: enum inside a List<>\ also becomes oneOf`
• with option: enum with no annotations stays as bare enum

All 55 tests pass.

[igordayen]

@sumitvairagar - thanks for the contribution.
Could you please sign the commit?
Please use uppercase in the PR description in the future (the PR description was generated by a coding agent, I assume)
I will reserve time for a thorough manual review and allocate the milestone accordingly
looping @LiveNathan