PRs with deleted heads fail validation

[trag1c]

Hi, first of all great project, thank you for this! ❤️
_{(far better experience than the most popular Python option 👀)}

It looks like GitHub changed the way they give info about PRs, and .head can now be null. From some quick testing it seems like this specifically affects PRs where head is a deleted fork, e.g. ghostty-org/ghostty#533:

from githubkit import GitHub

gh = GitHub(...)

gh.rest.pulls.get("ghostty-org", "ghostty", 533).parsed_data
# 1 validation error for PullRequest
# head.repo
#   Input should be an object [type=model_type, input_value=None, input_type=NoneType]
#     For further information visit https://errors.pydantic.dev/2.11/v/model_type

[yanyongyu]

Thanks for your report 👍🏻 . Missing to change the repo field in PullRequest schema.

Related to #56, github/rest-api-description#1811 .