[ci]: reduce MG3 SSIM test resolution to 352x640 (~4x faster)

Lowers MATRIXGAME3_PARAMS height/width from 720x1280 to 352x640 in the
SSIM regression test (matches MatrixGame 2.0 convention from the
MatrixGame2SamplingParam defaults). MATRIXGAME3_FULL_QUALITY_PARAMS
remains 720x1280 — it derives from SamplingParam.from_pretrained and
covers the full-quality tier separately.

Compute reduction: ~4x vs 720p (352*640 / 720*1280 ≈ 0.24). Brings the
SSIM test within typical L40S CI runtime budget so a HF reference can be
seeded against this resolution.

Followup to PR #1201 review:
#1201 (comment)

Author: SolitaryThinker

fastvideo/tests/ssim/test_matrixgame3_similarity.py

@@ -37,8 +37,8 @@
 MATRIXGAME3_PARAMS = {
     "num_gpus": 1,
     "model_path": "FastVideo/Matrix-Game-3.0-Base-Distilled-Diffusers",
-    "height": 720,
-    "width": 1280,
+    "height": 352,
+    "width": 640,
     "num_frames": 57,
     "num_inference_steps": 3,
     "guidance_scale": 5.0,