fix(sam21): broader httpx catch + tighter test assertions

Catch httpx.RequestError so DecodingError etc. trigger the bbox fallback
instead of propagating raw. Lock the intrinsics dict shape in the
happy-path test, drop unused imports.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

tests/test_sam21_client.py

@@ -1,8 +1,7 @@
 """Unit tests for Sam21Client."""
 from __future__ import annotations
 
-import json
-from unittest.mock import MagicMock, patch
+from unittest.mock import patch
 
 import httpx
 import numpy as np
@@ -56,6 +55,10 @@ def test_segment_pointcloud_happy_path():
     assert body["return_mask"] is False
     assert isinstance(body["rgb_png_base64"], str) and len(body["rgb_png_base64"]) > 0
     assert isinstance(body["depth_png_base64"], str) and len(body["depth_png_base64"]) > 0
+    assert body["intrinsics"] == {
+        "fx": 600.0, "fy": 600.0, "cx": 320.0, "cy": 240.0,
+        "width": 640, "height": 480,
+    }
     # Output is in meters: mm/1000.
     assert out.shape == (2, 3)
     assert out.dtype == np.float64