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/queries/chats.sql

@@ -367,6 +367,7 @@ INSERT INTO chat_diff_statuses (
     author_login,
     author_avatar_url,
     base_branch,
+    head_branch,
     pr_number,
     commits,
     approved,
@@ -386,6 +387,7 @@ INSERT INTO chat_diff_statuses (
     sqlc.narg('author_login')::text,
     sqlc.narg('author_avatar_url')::text,
     sqlc.narg('base_branch')::text,
+    sqlc.narg('head_branch')::text,
     sqlc.narg('pr_number')::integer,
     sqlc.narg('commits')::integer,
     sqlc.narg('approved')::boolean,
@@ -406,6 +408,7 @@ SET
     author_login = EXCLUDED.author_login,
     author_avatar_url = EXCLUDED.author_avatar_url,
     base_branch = EXCLUDED.base_branch,
+    head_branch = EXCLUDED.head_branch,
     pr_number = EXCLUDED.pr_number,
     commits = EXCLUDED.commits,
     approved = EXCLUDED.approved,