feat(gitsync): enrich PR status with author, base branch, review info (#23038)

## Summary

Adds 7 new fields to the PR status stored by gitsync, all sourced from
the existing GitHub API calls (**zero additional HTTP requests**):

| Field | Source | Purpose |
|---|---|---|
| `author_login` | `pull.user.login` | PR author username |
| `author_avatar_url` | `pull.user.avatar_url` | PR author avatar for UI
|
| `base_branch` | `pull.base.ref` | Target branch (e.g. `main`) |
| `pr_number` | `pull.number` | Explicit PR number |
| `commits` | `pull.commits` | Number of commits in PR |
| `approved` | Derived from reviews | True when ≥1 approved, no
outstanding changes requested |
| `reviewer_count` | Derived from reviews | Distinct reviewers with a
decisive state |

## Changes

- **`gitprovider/gitprovider.go`**: Added 7 fields to `PRStatus` struct.
- **`gitprovider/github.go`**: Expanded the anonymous struct in
`FetchPullRequestStatus` to decode new JSON fields. Replaced
`hasOutstandingChangesRequested()` with `summarizeReviews()` returning a
`reviewStats` struct with `changesRequested`, `approved`, and
`reviewerCount`.
- **Migration 000434**: Adds 7 columns to `chat_diff_statuses`.
- **`queries/chats.sql`**: Updated `UpsertChatDiffStatus`
INSERT/VALUES/ON CONFLICT.
- **`gitsync/gitsync.go`**: Maps new `PRStatus` fields into upsert
params.
- **`gitsync/worker.go`**: Maps new columns in row-to-model converter.
- **`codersdk/chats.go`**: Added fields to SDK `ChatDiffStatus` type.
- **`coderd/chats.go`**: Maps new DB fields in
`convertChatDiffStatus()`.
- Auto-generated: `models.go`, `queries.sql.go`, `dump.sql`,
`typesGenerated.ts`.

coderd/externalauth/gitprovider/gitprovider.go

@@ -79,6 +79,23 @@ type PRStatus struct {
 	// ChangesRequested is a convenience boolean: true if any
 	// reviewer's current state is "changes_requested".
 	ChangesRequested bool
+	// AuthorLogin is the login/username of the PR author.
+	AuthorLogin string
+	// AuthorAvatarURL is the avatar URL of the PR author.
+	AuthorAvatarURL string
+	// BaseBranch is the target branch the PR will merge into.
+	BaseBranch string
+	// PRNumber is the PR number (e.g. 1347).
+	PRNumber int
+	// Commits is the number of commits in the PR.
+	Commits int32
+	// Approved is true when at least one reviewer has approved
+	// and no reviewer has outstanding changes requested.
+	Approved bool
+	// ReviewerCount is the number of distinct reviewers who
+	// have left a decisive review (approved, changes_requested,
+	// or dismissed).
+	ReviewerCount int32
 	// FetchedAt is when this status was fetched.
 	FetchedAt time.Time
 }