feat(seedance): add dedicated Seedance resource to TypeScript and Python SDKs

[Copilot]

The Seedance API uses a structured content array input (not a plain prompt) and requires model — making it incompatible with the generic Video.generate() interface. This adds dedicated Seedance/AsyncSeedance resource classes to both SDKs, following the same pattern as Kling and Veo.

New resources

• typescript/src/resources/seedance.ts — Seedance class, SeedanceModel enum type (8 model values), SeedanceContentItem union type
• python/src/acedatacloud/resources/seedance.py — Seedance + AsyncSeedance classes with identical interface

Client wiring

• client.seedance exposed on both sync/async clients in TypeScript and Python
• SeedanceModel and SeedanceContentItem exported from TypeScript index

API surface

await client.seedance.generate({
  model: 'doubao-seedance-1-0-pro-250528',
  content: [
    { type: 'text', text: 'A kitten yawning at the camera.' },
    { type: 'image_url', image_url: { url: 'https://...' }, role: 'first_frame' },
  ],
  resolution: '720p',
  ratio: '16:9',
  duration: 5,
});

Supports all optional fields from the spec: resolution, ratio, duration, frames, seed, camerafixed, watermark, generate_audio, return_last_frame, execution_expires_after, callback_url, async.