fix(coderd): refresh OAuth token before GitHub API calls in chat diff (#22415)

## Problem

`resolveChatGitHubAccessToken` reads the `OAuthAccessToken` directly
from the database without refreshing it. When the token expires, GitHub
returns "bad credentials" and the chat diff features break.

## Fix

Call `config.RefreshToken()` before returning the token — the same code
path used by `provisionerdserver` when handing tokens to provisioners.

- Builds a map of provider ID → `*externalauth.Config` during the
existing config iteration
- After fetching the `ExternalAuthLink` from the DB, calls
`cfg.RefreshToken()` if a matching config exists
- On refresh failure, falls through to the existing token (GitHub tokens
without expiry still work) with a debug log

coderd/chats.go

@@ -26,6 +26,7 @@ import (
 	"github.com/coder/coder/v2/coderd/database"
 	"github.com/coder/coder/v2/coderd/database/db2sdk"
 	"github.com/coder/coder/v2/coderd/database/dbauthz"
+	"github.com/coder/coder/v2/coderd/externalauth"
 	"github.com/coder/coder/v2/coderd/httpapi"
 	"github.com/coder/coder/v2/coderd/httpapi/httperror"
 	"github.com/coder/coder/v2/coderd/httpmw"
@@ -1442,6 +1443,9 @@ func (api *API) resolveChatGitHubAccessToken(
 	ctx context.Context,
 	userID uuid.UUID,
 ) string {
+	// Build a map of provider ID -> config so we can refresh tokens
+	// using the same code path as provisionerdserver.
+	ghConfigs := make(map[string]*externalauth.Config)
 	providerIDs := []string{"github"}
 	for _, config := range api.ExternalAuthConfigs {
 		if !strings.EqualFold(
@@ -1451,6 +1455,7 @@ func (api *API) resolveChatGitHubAccessToken(
 			continue
 		}
 		providerIDs = append(providerIDs, config.ID)
+		ghConfigs[config.ID] = config
 	}
 
 	seen := map[string]struct{}{}
@@ -1471,6 +1476,24 @@ func (api *API) resolveChatGitHubAccessToken(
 			continue
 		}
 
+		// Refresh the token if there is a matching config, mirroring
+		// the same code path used by provisionerdserver when handing
+		// tokens to provisioners.
+		if cfg, ok := ghConfigs[providerID]; ok {
+			refreshed, refreshErr := cfg.RefreshToken(ctx, api.Database, link)
+			if refreshErr != nil {
+				api.Logger.Debug(ctx, "failed to refresh external auth token for chat diff",
+					slog.F("provider_id", providerID),
+					slog.F("user_id", userID),
+					slog.Error(refreshErr),
+				)
+				// Fall through — the existing token may still work
+				// (e.g. GitHub tokens with no expiry).
+			} else {
+				link = refreshed
+			}
+		}
+
 		token := strings.TrimSpace(link.OAuthAccessToken)
 		if token != "" {
 			return token