feat: add head_branch to pull request diff status (#23076)

Adds the `head_branch` field (the source/feature branch name of a PR) to
the diff status pipeline. Previously only `base_branch` (target branch)
and the head commit SHA were captured from the GitHub API, but not the
head branch name itself.

## Changes

- **Migration 438**: Add `head_branch` nullable TEXT column to
`chat_diff_statuses`
- **gitprovider**: Parse `head.ref` from the GitHub API response
(alongside `head.sha`) and add `HeadBranch` to `PRStatus`
- **gitsync**: Wire `HeadBranch` through `refreshOne()` into the DB
upsert params
- **worker**: Map `HeadBranch` in `chatDiffStatusFromRow()`
- **coderd**: Convert `HeadBranch` in `convertChatDiffStatus()`
- **codersdk**: Expose as `head_branch` (`*string`, omitempty) in
`ChatDiffStatus` API response
- **Tests**: Updated `github_test.go` pull JSON fixtures and assertions

codersdk/chats.go

@@ -633,6 +633,7 @@ type ChatDiffStatus struct {
 	AuthorLogin      *string    `json:"author_login,omitempty"`
 	AuthorAvatarURL  *string    `json:"author_avatar_url,omitempty"`
 	BaseBranch       *string    `json:"base_branch,omitempty"`
+	HeadBranch       *string    `json:"head_branch,omitempty"`
 	PRNumber         *int32     `json:"pr_number,omitempty"`
 	Commits          *int32     `json:"commits,omitempty"`
 	Approved         *bool      `json:"approved,omitempty"`