feat(amux): add Doubao-Seedance video provider (async media proxy)

[duanbing]

Adds a new amux media provider so RouterBase can route ByteDance Doubao-Seedance 2.0 video through the gateway, mirroring the existing Novita media-proxy path.

What

• tensorzero-core/src/providers/amux.rs — AmuxProvider::infer_media_proxy: submit to amux's universal POST /v1/video/generations, poll GET /v1/video/generations/{task_id} until completed (root url) or failed (error.message), then fire the RouterBase media callback. AmuxRequestShape covers seedance2_text_to_video and seedance2_image_to_video (builds the universal {model, prompt, image?, seconds?, metadata{ratio,resolution,seed,…}} body). Auth via AMUX_API_KEY / AMUX_API_BASE (default https://api.amux.ai).
• variant/media.rs — new MediaProxyConfig::Amux arm (provider tag amux) wired into infer + validate.
• providers/mod.rs — register the module.

Structurally a close mirror of providers/novita.rs; the difference is amux's universal endpoints are fixed (model id rides in the request body via proxy.path) rather than Novita's per-model async/{model} URL path.

Verification

• cargo check -p tensorzero-core ✓, cargo clippy -p tensorzero-core ✓ (amux.rs: 0 warnings), cargo fmt ✓.

Consumer

RouterBase parent PR: routes source='amux' / proxy.provider="amux" variants here (regenerated tensorzero.toml), seeds the Seedance models, and ships user docs.

🤖 Generated with Claude Code

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