chore(typing): fix mypy>=1.14.0 warnings

[dangotbanned]

Resolves 2 new warnings that appeared after updating my env.

Warnings

tools/markup.py:71: error: Incompatible types in assignment (expression has type "str | list[dict[str, Any]]", variable has type "str")  [assignment]
            s = super().__call__(s)  # pyright: ignore[reportAssignmentType]
                ^~~~~~~~~~~~~~~~~~~
tools/markup.py:158: error: Incompatible return value type (got "str | list[dict[str, Any]]", expected "list[dict[str, Any]]")  [return-value]
        return tokens[0]
               ^~~~~~~~~
Found 2 errors in 1 file (checked 11 source files)

Additional

The second warning reminded me of (#3687 (comment)), so I've done some tidying up and opened a PR upstream

• docs: Replace redirect url in expressions.md vega#3996