Feature: native structured output refinements — strict mode, schema fixes

[igordayen]

Summary

See issue: #1966

• Strict mode: OpenAiNativeStructuredOutputConfigurer now calls .strict(true) on the OpenAI response format, enforcing the schema contract at the API level
• Schema fix: schemaType() in jsonSchemaSupport correctly handles array nodes
• Nullable union: `supports Java Optional
• Docs: NativeStructuredOutputMode KDoc clarifies that DISABLED opts out entirely
  DEFAULT policy description tightened

Java — Optional field example

class PersonWithAge {
    @JsonProperty(required = true)
    public String name;
    @JsonProperty(required = true)
    public int age;
    @JsonProperty(required = true)
    public Optional<String> nickname;  // maps to {"type":["string","null"]} in JSON Schema
}

Optional and Kotlin T? are both supported in DEFAULT and ENABLED modes. The field is included in required[] with a
["type","null"] union — OpenAI strict mode accepts this.

Known limitations (DEFAULT mode)

• Enums — not supported; fall back to prompt-based extraction
• Map<K,V> — not supported (produces additionalProperties, rejected by the compatibility check)
• Nested arrays of arrays — not supported (2D arrays fall through to else -> false)
• $ref, oneOf, anyOf, allOf, not — any schema using these keywords is rejected; affects sealed classes, polymorphic types, and Jackson subtypes
• Optional<Array/List> — nullable arrays not supported in the union check (else -> false in isValidNullableUnion)
• ENABLED mode bypasses all of the above — no compatibility check, schema is sent as-is; the provider may reject it

Testing

• NativeStructuredOutputSupportTest — nullable union and array compatibility
• OpenAiNativeStructuredOutputConfigurerTest — strict mode assertion
• jsonSchemaSupportTest — schema type correctness
• OpenAiNativeStructuredOutputIT — live OpenAI (requires OPENAI_API_KEY)

[sonarqubecloud]

Quality Gate passed

Issues
2 New issues
0 Accepted issues

Measures
0 Security Hotspots
92.2% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud

[igordayen]

@stefanks-px - could you please validate, thank you