[ty] Pydantic: Fix float conversion in unions (#26655)
## Summary
`float`s that were embedded in unions were not recognized by our lax
mode mapping (since we were only looking for the expanded `float |
int`), so they were mapped to `Any` instead of `LaxFloat`, which lead to
overly permissive lax types.
closes https://github.com/astral-sh/ty/issues/3946
## Test Plan
Added a regression test
