fix(audio): treat DIARIZED_JSON as a JSON response format (#652)

[spor3006]

Fixes #652.

Problem

AudioResponseFormat.isJson() was missing a DIARIZED_JSON -> true branch:

internal fun isJson(): Boolean =
    when (this) {
        JSON -> true
        TEXT -> false
        SRT -> false
        VERBOSE_JSON -> true
        VTT -> false
        else -> false   // ← DIARIZED_JSON silently fell through to false
    }

When a caller requested response_format = DIARIZED_JSON from audio.transcriptions().create(...), the SDK treated the response as plain text. The entire diarized JSON payload was stuffed into TranscriptionCreateResponse.transcription.text, leaving TranscriptionCreateResponse.diarized empty. The only workaround was to manually re-parse the raw text field through Jackson:

new ObjectMapper().readValue(
    response.transcription().get().text(),
    TranscriptionDiarized.class
);

Fix

Two small changes:

1. Add the missing case to AudioResponseFormat.isJson():

   DIARIZED_JSON -> true

2. Add AudioResponseFormatTest that enumerates every value of AudioResponseFormat.Known explicitly and asserts the expected isJson() return value. Because the test exhaustively switches on Known, adding any new format to the enum will fail compilation of the test until the test author explicitly chooses true or false for it. The same class of bug cannot regress.

Tests

$ ./gradlew :openai-java-core:test --tests "com.openai.models.audio.AudioResponseFormatTest"

AudioResponseFormatTest > isJson_returnsTrue_forEveryJsonResponseFormat PASSED
AudioResponseFormatTest > diarizedJson_isReportedAsJson PASSED

Verified the second test (diarizedJson_isReportedAsJson) fails on main without the production change — the response would be reported as non-JSON.

Scope

• Only AudioResponseFormat.kt (one line) and the new test file change. No public API surface change.
• The TranscriptionCreateResponse.isValid() follow-up symptom noted in the issue (server-side response missing required fields per the OpenAPI spec) is left out — that's a spec/server question, not an SDK bug.

Fixes #652