This commit upgrades our Mock Servlet classes for Servlet 6.1 support:
* the read/write `ByteBuffer` variants for `ServletInputStream` and
`ServletOutputStream` were not added as the default implementation
matches well the testing use case.
* Implement the session accessor with a simple lambda. Our mocks do not
simulate the scheduling of request/response processing on different
threads.
* Ensure that the response content length can only be written before the
response is committed. Calling those methods after commit is a no-op,
per specification.
Closes gh-33749
5e08a88219