Honor RoundTripper contract in middleware; validate incoming X-Request-Id

BearerAuth, BasicAuth and DefaultHeaders mutated the caller's request, which
violates the RoundTripper contract and risks races on shared/retried requests;
clone before writing headers (matching RequestID). Validate the incoming
X-Request-Id (length and character set) before propagating it to logs and the
response header, preventing log forging and header splitting from a
client-controlled value.

middleware/auth.go

@@ -15,6 +15,9 @@ func BearerAuth(tokenFunc func(ctx context.Context) (string, error)) Middleware
 			if err != nil {
 				return nil, err
 			}
+			// RoundTrippers must not mutate the caller's request; clone before
+			// setting headers (req.Clone is shallow + a header copy).
+			req = req.Clone(req.Context())
 			req.Header.Set("Authorization", "Bearer "+token)
 			return next.RoundTrip(req)
 		})
@@ -26,6 +29,7 @@ func BearerAuth(tokenFunc func(ctx context.Context) (string, error)) Middleware
 func BasicAuth(username, password string) Middleware {
 	return func(next http.RoundTripper) http.RoundTripper {
 		return RoundTripperFunc(func(req *http.Request) (*http.Response, error) {
+			req = req.Clone(req.Context())
 			req.SetBasicAuth(username, password)
 			return next.RoundTrip(req)
 		})