Fix router media capability selection and Anthropic thinking replay

[Kaguya-19]

Summary

• Prevent router from selecting text-only models for requests containing image, PDF, or audio input.
• Make model request validation reject unsupported media instead of silently replacing it with placeholders.
• Preserve Anthropic thinking signatures when replaying assistant thinking blocks.

Changes

• Added media requirement detection for canonical messages and router fallback filtering based on multimodal.input.
• Added mediaCapabilityRerouted mutation metadata for router decisions corrected by media capability checks.
• Removed unsupported media downgrading from model request validation.
• Preserved Anthropic thinking.signature in request building and non-streaming response parsing.

Verification

• Ran focused TypeScript checks for modified router/model files.
• Ran focused local tests for media routing and Anthropic thinking signature handling.
• Full build was not used as final verification because existing untracked tests in this workspace reference outdated APIs and fail independently.