fix(presets): validate required manifest mappings (#3898)

Assisted-by: GitHub Copilot (model: GPT-5.6 Sol, autonomous)

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

src/specify_cli/presets/__init__.py

@@ -299,6 +299,12 @@ class PresetManifest:
                 f"(expected {self.SCHEMA_VERSION})"
             )
 
+        for section in ("preset", "requires", "provides"):
+            if not isinstance(self.data[section], dict):
+                raise PresetValidationError(
+                    f"Invalid {section}: expected a mapping"
+                )
+
         # Validate preset metadata
         pack = self.data["preset"]
         for field in ["id", "name", "version", "description"]: