fix: skip JSON pre-parsing for str union annotations in pre_parse_json

[adityuhkapoor]

Summary

pre_parse_json() uses field_info.annotation is not str to decide
whether to attempt json.loads() on string values. For str | None,
the annotation is not identical to str, so the check passes and
strings like '{"key": "val"}' get parsed into dicts — then rejected
by Pydantic validation.

Replace the identity check with a helper that skips pre-parsing when
the union only contains simple types. Complex unions like
list[str] | None still get pre-parsed.

Closes #1873

Test plan

• Regression test for str | None preserving JSON strings
• Verifies complex unions still pre-parse
• All existing tests pass