fix(oauth): narrow exp_time before subtraction (mypy 1.14 + pyjwt 2.13)

Previous fix wrapped exp_time in bool() to satisfy mypy's [return-value]
rule. That didn't narrow the type, so mypy then flagged the
`(exp_time - buffer_time)` expression with `[operator]` "Unsupported
operand types for - ('None' and 'int')" — the short-circuit at runtime
is correct, but mypy can't track narrowing through a bool() call.

Replace with explicit `if exp_time is None: return False`. Same runtime
semantics (None and falsy exp_time both produce False), cleaner reading,
and mypy narrows the rest of the function to int after the guard.

Co-authored-by: Isaac
Signed-off-by: Vikrant Puppala <vikrant.puppala@databricks.com>

Author: vikrantpuppala

src/databricks/sql/auth/oauth.py

@@ -43,9 +43,11 @@ def is_expired(self) -> bool:
                 self.access_token, options={"verify_signature": False}
             )
             exp_time = decoded_token.get("exp")
+            if exp_time is None:
+                return False
             current_time = time.time()
             buffer_time = 30  # 30 seconds buffer
-            return bool(exp_time) and (exp_time - buffer_time) <= current_time
+            return (exp_time - buffer_time) <= current_time
         except Exception as e:
             logger.error("Failed to decode token: %s", e)
             raise e