Issue #3572139: Add anthropic-ai/sdk dependency and native client service

Carlos Ospinarequested to merge
issue/ai_provider_anthropic-3572139:3572139-add-anthropic-aisdk-dependency into 1.3.x

Add the native Anthropic PHP SDK alongside the existing OpenAI compatibility layer, creating the foundation for native API features (effort, thinking, caching, structured outputs) in future issues.

  • Add anthropic-ai/sdk ^0.5 and php-http/guzzle7-adapter ^1.0 dependencies
  • Add AnthropicNativeClient service wrapping SDK's request() escape hatch
  • Add chat() routing override with requiresNativeApi() gate
  • Add nativeChat(), buildNativePayload(), parseNativeResponse() methods
  • Add handleApiException() override preserving parent rate-limit detection
  • Add 18 unit tests (36 assertions) covering routing, payload, parsing
  • Native path currently gated (requiresNativeApi returns FALSE until Phase 1)

Co-Authored-By: Claude Opus 4.6 noreply@anthropic.com

Closes #3572139

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