Detect exhaustiveness in partially-defined variable checks

For the partially defined check (--enable-error-code partially-defined), mypy generates a false-positive in the following case:

def f1(x: int | str) -> int:
  if isinstance(x, int):
    y = 1
  elif isinstance(x, str): 
    y = 2 
  return y # error: Name "y" may be undefined

The same applies to match statements:

def f(x: int | str) -> int:
  match x: 
    case int():
      y = 1 
    case str():
      y = 2 
  return y # error: Name "y" may be undefined

It's likely that mypy already detects this somewhere, since it doesn't complain about the missing return:

def f3(x: int | str) -> int:
  if isinstance(x, int):
    return 1
  elif isinstance(x, str): 
    return 2

Start by tracing mypy's partially-defined variable analysis and the existing exhaustiveness logic used for missing-return checks. Add regression coverage for the isinstance and match examples, then verify that exhaustive branches no longer report y as possibly undefined while genuinely non-exhaustive checks still do.