Raise on unexpected operands in the formula operators

The `match` in each binary operator covers all four combinations of its two
operands, plus the ones where an operand is `None`, and was followed by an
unreachable `return None`.  mypy can't narrow tuple items across `case`
clauses (see python/mypy#12364), so it can't prove the `match` exhaustive no
matter how many cases are written.

Turn that trailing `return None` into the explicit last case, and raise
instead of returning, so an operand of an unexpected type fails early rather
than silently evaluating the whole formula to nothing: type hints are not
enforced at runtime.  `assert_never()` can't be used here, as mypy doesn't
narrow the subject to `Never`.

Signed-off-by: Leandro Lucarella <luca-frequenz@llucax.com>

Author: llucax

src/frequenz/sdk/timeseries/formulas/_ast.py

@@ -177,7 +177,11 @@ async def evaluate(self) -> Sample[QuantityT] | QuantityT | None:
                 )
             case (None, _) | (_, None):
                 return None
-        return None
+            case unexpected:
+                # The cases above are exhaustive, but mypy can't narrow tuple patterns
+                # (see python/mypy#12364), so it neither sees that nor accepts
+                # assert_never() here.
+                raise AssertionError(f"Unexpected operands: {unexpected!r}")
 
     @override
     def format(self, wrap: bool = False) -> str:
@@ -254,7 +258,11 @@ async def evaluate(self) -> Sample[QuantityT] | QuantityT | None:
                 )
             case (None, _) | (_, None):
                 return None
-        return None
+            case unexpected:
+                # The cases above are exhaustive, but mypy can't narrow tuple patterns
+                # (see python/mypy#12364), so it neither sees that nor accepts
+                # assert_never() here.
+                raise AssertionError(f"Unexpected operands: {unexpected!r}")
 
     @override
     def format(self, wrap: bool = False) -> str:
@@ -331,7 +339,11 @@ async def evaluate(self) -> Sample[QuantityT] | QuantityT | None:
                 )
             case (None, _) | (_, None):
                 return None
-        return None
+            case unexpected:
+                # The cases above are exhaustive, but mypy can't narrow tuple patterns
+                # (see python/mypy#12364), so it neither sees that nor accepts
+                # assert_never() here.
+                raise AssertionError(f"Unexpected operands: {unexpected!r}")
 
     @override
     def format(self, wrap: bool = False) -> str:
@@ -406,7 +418,11 @@ async def evaluate(self) -> Sample[QuantityT] | QuantityT | None:
                 )
             case (None, _) | (_, None):
                 return None
-        return None
+            case unexpected:
+                # The cases above are exhaustive, but mypy can't narrow tuple patterns
+                # (see python/mypy#12364), so it neither sees that nor accepts
+                # assert_never() here.
+                raise AssertionError(f"Unexpected operands: {unexpected!r}")
 
     @override
     def format(self, wrap: bool = False) -> str: