refactor(diffusion): migrate bridges to model builders

[yaoyu-33]

Summary

• migrate Flux, LLaDA 1.5, Nemotron Labs Diffusion, and Wan bridges to serializable ModelConfig plus standalone ModelBuilder contracts
• move official diffusion recipes off model-provider construction paths
• preserve exact legacy transformer defaults and compatibility behavior, including Flux openai_gelu and Nemotron block sizing
• extend registered-bridge and provider-free consumer audits to diffusion

Stack

• Base: refactor(models): migrate AutoBridge to model builders #4545
• This PR contains only diffusion-specific migration changes and tests.

Validation

• uvx pre-commit run --all-files
• two independent subagent review passes completed; all confirmed findings fixed
• EOS targeted unit tests are running

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[yaoyu-33]

/ok to test afaeece