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

coderd/database/models.go

@@ -4053,6 +4053,7 @@ type ChatDiffStatus struct {
 	Commits          sql.NullInt32  `db:"commits" json:"commits"`
 	Approved         sql.NullBool   `db:"approved" json:"approved"`
 	ReviewerCount    sql.NullInt32  `db:"reviewer_count" json:"reviewer_count"`
+	HeadBranch       sql.NullString `db:"head_branch" json:"head_branch"`
 }
 
 type ChatFile struct {