jimeng: add Volcengine Jimeng (即梦 AI) video provider with V4 signing

Implements the Jimeng/Volcengine part of issue #249, as agreed with
@xucailiang (who is handling the Kling provider separately).

Adds a first-class Jimeng video provider that calls the Volcengine
visual API directly (visual.volcengineapi.com) using HMAC-SHA256 V4
request signing with IAM AK/SK credentials. This is the first provider
in OpenMontage to use V4 signing (all others use Bearer token auth).

API flow: POST CVSync2AsyncSubmitTask -> poll CVSync2AsyncGetResult ->
download video_url.

Features:
- Text-to-video and image-to-video (Jimeng 3.0 Pro)
- Configurable frame count (121=5s, 241=10s at 24fps)
- Aspect ratio selection (16:9, 9:16, 1:1, etc.)
- Seed for reproducibility
- Full V4 HMAC-SHA256 request signing (not Bearer token)
- Error handling with Jimeng code 10000 success convention
- API key redaction in error messages (both env vars, no empty-string bug)

Env vars: VOLC_ACCESSKEY + VOLC_SECRETKEY (IAM AK/SK pair).
Idempotency keys include all output-affecting fields.

Files:
- tools/video/jimeng_video.py — new tool (V4 signing + submit/poll/download)
- tests/contracts/test_jimeng_video.py — 46 contract tests (no AK/SK needed)
- .env.example — VOLC_ACCESSKEY + VOLC_SECRETKEY
- docs/PROVIDERS.md — Volcengine Jimeng provider section

End-to-end tested with real Volcengine IAM credentials: generated a
1920x1088 H.264 5.04s video, ffprobe verified.

Test results:
  python -m pytest tests/contracts/test_jimeng_video.py -q  # 46 passed

.env.example

@@ -32,6 +32,8 @@ SUNO_API_KEY=                # Suno AI music generation (full songs, instrumenta
 # --- Video Generation ---
 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
 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)