Add Anthropic as a third LLM provider with Claude Opus 4.6, Sonnet 4.5,
and Haiku 4.5 models. Extend Google provider with Gemini 3.1 Pro.
Includes full API route support with multimodal content blocks,
provider settings UI, temperature clamping (Anthropic max 1.0),
header builder plumbing, and comprehensive test coverage.
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>