[ty] Pydantic: Add support for `validation_alias` (#26629)
## Summary
If `validation_alias` is specified, it[ takes precedence over `alias`
](https://pydantic.dev/docs/validation/latest/concepts/fields#field-aliases)
for purposes of the constructor signature, so here, we implement this by
simply overriding the `alias` attribute on the field.
## Test Plan
Update Markdown tests.
