feat(multimodal): audio input via OpenAI input_audio content parts

[vaiju1981]

Summary

Add audio input to the typed multimodal API, extending the vision work to audio
(llama.cpp discussion #13759: Ultravox /
Qwen2-Audio / Qwen2.5-Omni). No native/JNI change is needed — upstream b9739 already decodes the
OpenAI input_audio content part (server-common.cpp) into the same media buffer vision uses, which
the JNI bridge already threads to mtmd's audio pipeline (mtmd_support_audio /
mtmd_bitmap_init_from_audio). The only gap was the Java typed API:

• ContentPart — new INPUT_AUDIO kind + inputAudio(byte[], "wav"|"mp3") / audioFile(Path)
  factories (extension → format), with base64 data + format accessors.
• ParameterJsonSerializer.buildMessages emits
  {"type":"input_audio","input_audio":{"data","format"}}; ChatMessage.concatText already skips
  non-text parts, so getContent() is unaffected.
• LlamaModel.supportsAudio() (parallel to supportsVision(); ModelMeta.supportsAudio already
  existed, fed by the native meta's modalities.audio).

The OpenAI server needs no change — audio content parts already round-trip verbatim through
/v1/chat/completions.

Test plan

• Affected tests pass locally — ContentPart audio factories + format validation, a ChatRequest
  serializer test asserting the input_audio JSON shape (50 unit tests total); Spotless + Javadoc clean.
• CI is green on this branch
• Docs — README "Vision / Multimodal Chat" audio example + system-property rows; CLAUDE.md property
  table + run command.

Related issues / PRs

Implements audio input per llama.cpp discussion #13759; extends the vision-input work.

Note for reviewer

AudioInputIntegrationTest (Ultravox / Qwen2.5-Omni) is gated and self-skips without the three
audio.* system properties (model / mmproj / clip), exactly like MultimodalIntegrationTest. The audio
model download is intentionally not added to CI (Ultravox is large and the test self-skips) — it's
documented as locally/CI-runnable. So the serialization path is unit-tested; the real-model audio path
is gated and unrun in CI.

Checklist

• I have read CONTRIBUTING.md and CODE_OF_CONDUCT.md
• My commits follow Conventional Commits
• No security-sensitive changes (if there are, I have notified the maintainer privately per SECURITY.md)