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>

src/presa_mano/clients/sam21.py

@@ -63,7 +63,7 @@ class Sam21Client:
         try:
             with httpx.Client(timeout=self.timeout_s) as c:
                 r = c.post(url, json=body)
-        except (httpx.ConnectError, httpx.TimeoutException, httpx.NetworkError) as e:
+        except httpx.RequestError as e:
             raise Sam21Unavailable(f"sam21 unreachable: {e}") from e
 
         # Try to parse the envelope even on non-2xx, because SAM21 always