Fix raw_post raising when rack.input is nil

Starting in Rack 3.1, rack.input is optional, so `read_body_stream`
(used by `raw_post`) can no longer call `read` unconditionally.

actionpack/CHANGELOG.md

@@ -1,3 +1,7 @@
+*   Fix `Request#raw_post` raising `NoMethodError` when `rack.input` is `nil`.
+
+    *Hartley McGuire*
+
 *   Add `:wasm_unsafe_eval` mapping for `content_security_policy`
 
     ```ruby

actionpack/lib/action_dispatch/http/request.rb

@@ -466,11 +466,13 @@ def default_session
       end
 
       def read_body_stream
-        reset_stream(body_stream) do
-          if headers.key?("Transfer-Encoding")
-            body_stream.read # Read body stream until EOF if "Transfer-Encoding" is present
-          else
-            body_stream.read(content_length)
+        if body_stream
+          reset_stream(body_stream) do
+            if headers.key?("Transfer-Encoding")
+              body_stream.read # Read body stream until EOF if "Transfer-Encoding" is present
+            else
+              body_stream.read(content_length)
+            end
           end
         end
       end

actionpack/test/dispatch/request_test.rb

@@ -1199,6 +1199,12 @@ class RequestParameters < BaseRequestTest
     assert_not_nil e.cause
     assert_equal e.cause.backtrace, e.backtrace
   end
+
+  test "raw_post does not raise when rack.input is nil" do
+    request = stub_request
+
+    assert_nil request.raw_post
+  end
 end
 
 class RequestParameterFilter < BaseRequestTest