pyproject.toml: Enable mypy.disallow_incomplete_defs

Add disallow_incomplete_defs = true to the mypy configuration. This
requires all function definitions to have complete parameter and return
type annotations, catching partially-annotated function signatures.

Assisted-by: unsloth/Qwen3.6-35B-A3B-GGUF:IQ4_NL and pi.dev 0.81.1
Signed-off-by: Jan Lindemann <jan@janware.com>

conf/templates/pyproject.toml

@@ -16,6 +16,8 @@
 
   warn_return_any = true
 
+  disallow_incomplete_defs = true
+
   enable_error_code = [
     "explicit-override",
     "ignore-without-code",