feat: add Tencent Hunyuan cloud video provider via TokenHub API

Introduce a new video generation provider backed by the Tencent TokenHub
API (tokenhub.tencentmaas.com), an OpenAI-compatible gateway for Tencent
Hunyuan video models with simple Bearer-token auth.

- Add hunyuan_cloud_video tool (submit → poll → download) supporting
  both text-to-video (hy-video-1.5) and image-to-video (yt-video-2.0)
- Add env vars: TENCENT_TOKENHUB_API_KEY, TENCENT_TOKENHUB_MODEL
- Add contract tests for the new tool
- Document setup, API flow, model pricing, and schema constraints in
  PROVIDERS.md
- Update provider tables and capability matrix throughout docs

.env.example

@@ -51,7 +51,10 @@ SUNO_API_KEY=                # Suno AI music generation (full songs, instrumenta
 HEYGEN_API_KEY=              # HeyGen API (VEO, Sora, Runway, Kling, Seedance via single key)
 RUNWAY_API_KEY=              # Runway Gen-4 (direct API, alternative to fal.ai routing)
 VOLC_ACCESSKEY=              # Volcengine Jimeng (即梦 AI) video generation via official API (HMAC-SHA256 V4 signing)
-VOLC_SECRETKEY=             # Secret Access Key paired with VOLC_ACCESSKEY. Get both at https://console.volcengine.com/iam/keymanage
+VOLC_SECRETKEY=              # Secret Access Key paired with VOLC_ACCESSKEY. Get both at https://console.volcengine.com/iam/keymanage
+TENCENT_TOKENHUB_API_KEY=    # Tencent Hunyuan (腾讯混元) video generation via TokenHub API (Bearer token)
+                              # Get it at https://console.cloud.tencent.com/tokenhub
+# TENCENT_TOKENHUB_MODEL=    # Optional: override default TokenHub model (hy-video-1.5 / yt-video-2.0)
 VIDEO_GEN_LOCAL_ENABLED=     # Set to "true" for local video gen (needs GPU + diffusers)
 VIDEO_GEN_LOCAL_MODEL=       # Local model: wan2.1-1.3b, wan2.1-14b, hunyuan-1.5, ltx2-local, cogvideo-5b
 MODAL_LTX2_ENDPOINT_URL=    # Modal self-hosted LTX-2 endpoint (optional)