fix: use chunked read for extension manifest hash (#3841)

Replace unbounded f.read() with chunked iteration to prevent excessive
memory allocation on large or corrupted manifest files. Matches the
pattern used in integrations/manifest.py _sha256().

src/specify_cli/extensions/__init__.py

@@ -566,8 +566,11 @@ class ExtensionManifest:
 
     def get_hash(self) -> str:
         """Calculate SHA256 hash of manifest file."""
+        h = hashlib.sha256()
         with open(self.path, "rb") as f:
-            return f"sha256:{hashlib.sha256(f.read()).hexdigest()}"
+            for chunk in iter(lambda: f.read(8192), b""):
+                h.update(chunk)
+        return f"sha256:{h.hexdigest()}"
 
 
 class ExtensionRegistry: