refactor: remove TENCENT_TOKENHUB_MODEL env var, relocate Hunyuan entry in .env.example

The env var added complexity without meaningful benefit — explicit model
selection via the "model" input parameter is sufficient for both image and
video TokenHub tools.

- tools/video/hunyuan_cloud_video.py: drop env var fallback from
  _resolve_model(), remove mention from install_instructions
- tests/contracts/test_hunyuan_cloud_video.py: remove
  test_resolve_model_from_env and test_resolve_model_input_overrides_env
- .env.example: move TENCENT_TOKENHUB_API_KEY to a dedicated "Tencent
  Hunyuan TokenHub API" section, drop TENCENT_TOKENHUB_MODEL comment,
  broaden description from "video generation" to generic "Tencent Hunyuan
  via TokenHub API"

.env.example

@@ -44,6 +44,10 @@ DOUBAO_SPEECH_VOICE_TYPE=    # Default Doubao speaker/voice type, e.g. zh_female
 DASHSCOPE_API_KEY=           # Qwen image gen (qwen-image-2.0-pro), TTS (qwen3-tts-flash), ASR with word timestamps (qwen3-asr-flash-filetrans)
                               # Get one at https://dashscope.aliyun.com/
 
+# --- Tencent Hunyuan TokenHub API ---
+TENCENT_TOKENHUB_API_KEY=     # Tencent Hunyuan (腾讯混元) via TokenHub API (Bearer token)
+                              # Get it at https://console.cloud.tencent.com/tokenhub
+
 # --- Music ---
 SUNO_API_KEY=                # Suno AI music generation (full songs, instrumentals, any genre)
 
@@ -52,9 +56,6 @@ HEYGEN_API_KEY=              # HeyGen API (VEO, Sora, Runway, Kling, Seedance vi
 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
-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)