feat : Spring AI 2.0 starter — chat, image, audio adapters

[omahjoub]

Summary

Adds spring-ai-starter module: thin adapter layer over the SB4 starter that
exposes the FanarClient as Spring AI's typed model SPIs. One dep, four beans —
applications use Spring AI's ChatClient, advisors, prompt templates, and reactive
streaming without touching the Fanar SDK directly.

Adapters:

• FanarChatModel (also satisfies StreamingChatModel) — maps Prompt/ChatOptions
  onto ChatRequest, bridges Flow.Publisher to Flux,
  silently drops TOOL messages and ProgressChunk/ToolCallChunk/ToolResultChunk
  (Fanar rejects user tools server-side).
• FanarImageModel — maps ImagePrompt onto ImageGenerationRequest, joins
  multi-message prompts with newlines, returns b64Json.
• FanarTextToSpeechModel (+ StreamingTextToSpeechModel) — Fanar TTS is one-shot;
  stream() wraps the call() result as a single-element Flux for API symmetry.
• FanarTranscriptionModel — reads bytes from Spring's Resource, infers
  Content-Type from filename extension, always requests text format.

FanarSpringAiAutoConfiguration registers all four as @ConditionalOnMissingBean
so users can override per slot. Activates only when both Spring AI and a
FanarClient bean are present.

Build wiring: pin spring-ai.version=2.0.0-M4 (aligned with SB4 / Spring 7),
import spring-ai-bom, add spring-milestones repo at parent level scoped to
milestones-only via releases.enabled=false.

Deferred:

• ModerationModel — shape mismatch (Spring AI expects 16-category booleans;
  Fanar returns continuous safety + culturalAwareness scores). Would always
  return Categories.isHate()=false etc., misleading consumers.
• EmbeddingModel — no Fanar embeddings endpoint exists. Documented gap;
  RAG users bring their own EmbeddingModel from spring-ai-openai, etc.

Type of change

• Bug fix
• New feature / capability
• New module
• Refactoring (no behavior change)
• Breaking change
• Documentation / tooling / CI only

Test plan

• mvn verify passes locally
• Live tests pass against the real Fanar API (if applicable, with FANAR_API_KEY set)
• GraalVM --self-test passes (if the SDK's reflective surface changed)