Add overloads to gen.multi(). Closes #3514.

[jrheard]

This diff resolves #3514 .

Repro file: https://github.com/jrheard/tornado-gen-repro/blob/main/main.py

When I install tornado's master branch and run mypy on that file, I get this output:

🚂  uv run mypy .
main.py:24: error: Argument 1 to "takes_list" has incompatible type "list[Any] | dict[Any, Any]"; expected "list[int]"  [arg-type]
main.py:25: error: Argument 1 to "takes_dict" has incompatible type "list[Any] | dict[Any, Any]"; expected "dict[str, int]"  [arg-type]
Found 2 errors in 1 file (checked 1 source file)

When I install this gen-multi-overload branch and run mypy on that file, I get this output:

🚂  uv run mypy .
Success: no issues found in 1 source file

This branch is clean on mypy 1.15.0:

🚂  mypy tornado
Success: no issues found in 75 source files