refactor: extract readBody helper to remove duplicate io.ReadAll calls (#1241)

The same pattern for reading an error response body appeared in three
places across the auth handlers:

- `internal/api/handlers/v0/auth/github_at.go` (twice)
- `internal/api/handlers/v0/auth/github_oidc.go` (once)

A shared `readBody` helper function added to `common.go` now handles
this in one place.
The `io` import is removed from both files since it is no longer used
directly there.

---------

Co-authored-by: Radoslav Dimitrov <radoslav@stacklok.com>

internal/database/postgres.go

@@ -198,7 +198,6 @@ func (db *PostgreSQL) ListServers(
 		whereConditions = append(whereConditions, cursorCondition)
 		args = append(args, cursorArgs...)
 	}
-	_ = argIndex // Silence unused variable warning
 
 	// Build the WHERE clause
 	whereClause := ""