fix: update sidebar diff status on WebSocket events (#23116)

## Problem

The sidebar diff status (PR icon, +additions/-deletions, file count) was
not updating in real-time. Users had to reload the page to see changes.

Two root causes:

1. **Frontend**: The `diff_status_change` WebSocket handler in
`AgentsPage.tsx` had an early `return` (line 398) that skipped
`updateInfiniteChatsCache`, so the sidebar's cache was never updated.
Even for other event types, the cache merge only spread `status` and
`title` — never `diff_status`.

2. **Server**: `publishChatPubsubEvent` in `chatd.go` constructed a
minimal `Chat` payload without `DiffStatus`, so even if the frontend
consumed the event, `updatedChat.diff_status` would be `undefined`.

## Fix

### Server (`coderd/chatd/chatd.go`)
- `publishChatPubsubEvent` now accepts an optional
`*codersdk.ChatDiffStatus` parameter; when non-nil it's set on the
outgoing `Chat` payload.
- `PublishDiffStatusChange` fetches the diff status from the DB,
converts it, and passes it through.
- Added `convertDBChatDiffStatus` (mirrors `coderd/chats.go`'s converter
to avoid circular import).
- All other callers pass `nil`.

### Frontend (`site/src/pages/AgentsPage/AgentsPage.tsx`)
- Removed the early `return` so `diff_status_change` events fall through
to the cache update logic.
- Added `isDiffStatusEvent` flag and spread `diff_status` into both the
infinite chats cache (sidebar) and the individual chat cache.

coderd/database/db2sdk/db2sdk.go

@@ -21,6 +21,7 @@ import (
 	agentproto "github.com/coder/coder/v2/agent/proto"
 	"github.com/coder/coder/v2/coderd/chatd/chatprompt"
 	"github.com/coder/coder/v2/coderd/database"
+	"github.com/coder/coder/v2/coderd/externalauth/gitprovider"
 	"github.com/coder/coder/v2/coderd/rbac"
 	"github.com/coder/coder/v2/coderd/rbac/policy"
 	"github.com/coder/coder/v2/coderd/render"
@@ -1164,3 +1165,86 @@ func nullInt64Ptr(v sql.NullInt64) *int64 {
 	value := v.Int64
 	return &value
 }
+
+// ChatDiffStatus converts a database.ChatDiffStatus to a
+// codersdk.ChatDiffStatus. When status is nil an empty value
+// containing only the chatID is returned.
+func ChatDiffStatus(chatID uuid.UUID, status *database.ChatDiffStatus) codersdk.ChatDiffStatus {
+	result := codersdk.ChatDiffStatus{
+		ChatID: chatID,
+	}
+	if status == nil {
+		return result
+	}
+
+	result.ChatID = status.ChatID
+	if status.Url.Valid {
+		u := strings.TrimSpace(status.Url.String)
+		if u != "" {
+			result.URL = &u
+		}
+	}
+	if result.URL == nil {
+		// Try to build a branch URL from the stored origin.
+		// Since this function does not have access to the API
+		// instance, we construct a GitHub provider directly as
+		// a best-effort fallback.
+		// TODO: This uses the default github.com API base URL,
+		// so branch URLs for GitHub Enterprise instances will
+		// be incorrect. To fix this, this function would need
+		// access to the external auth configs.
+		gp := gitprovider.New("github", "", nil)
+		if gp != nil {
+			if owner, repo, _, ok := gp.ParseRepositoryOrigin(status.GitRemoteOrigin); ok {
+				branchURL := gp.BuildBranchURL(owner, repo, status.GitBranch)
+				if branchURL != "" {
+					result.URL = &branchURL
+				}
+			}
+		}
+	}
+	if status.PullRequestState.Valid {
+		pullRequestState := strings.TrimSpace(status.PullRequestState.String)
+		if pullRequestState != "" {
+			result.PullRequestState = &pullRequestState
+		}
+	}
+	result.PullRequestTitle = status.PullRequestTitle
+	result.PullRequestDraft = status.PullRequestDraft
+	result.ChangesRequested = status.ChangesRequested
+	result.Additions = status.Additions
+	result.Deletions = status.Deletions
+	result.ChangedFiles = status.ChangedFiles
+	if status.AuthorLogin.Valid {
+		result.AuthorLogin = &status.AuthorLogin.String
+	}
+	if status.AuthorAvatarUrl.Valid {
+		result.AuthorAvatarURL = &status.AuthorAvatarUrl.String
+	}
+	if status.BaseBranch.Valid {
+		result.BaseBranch = &status.BaseBranch.String
+	}
+	if status.HeadBranch.Valid {
+		result.HeadBranch = &status.HeadBranch.String
+	}
+	if status.PrNumber.Valid {
+		result.PRNumber = &status.PrNumber.Int32
+	}
+	if status.Commits.Valid {
+		result.Commits = &status.Commits.Int32
+	}
+	if status.Approved.Valid {
+		result.Approved = &status.Approved.Bool
+	}
+	if status.ReviewerCount.Valid {
+		result.ReviewerCount = &status.ReviewerCount.Int32
+	}
+	if status.RefreshedAt.Valid {
+		refreshedAt := status.RefreshedAt.Time
+		result.RefreshedAt = &refreshedAt
+	}
+	staleAt := status.StaleAt
+	result.StaleAt = &staleAt
+
+	return result
+}