pyproject.toml: Enable mypy.exhaustive-match

Add the exhaustive-match error code to the mypy configuration. This
requires all match statements to be exhaustive, handling all possible
values of the matched expression.

The fix is to add 'case _: pass' to each match statement that is
intended to be non-exhaustive, making the non-exhaustiveness explicit.

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 +16,7 @@
     "possibly-undefined",
     "truthy-bool",
     "truthy-iterable",
+    "exhaustive-match",
   ]
 
 [tool.isort]