Increase test coverage for servlet specification compliance

Currently there are two ways of mapping request events to servlet requests. The classic way is covered by tests using `com.amazonaws.serverless.proxy.internal.testutils.AwsProxyRequestBuilder`, the new way using Spring Cloud is tested in `com.amazonaws.serverless.proxy.spring.AwsSpringHttpProcessingUtilsTests`. The latter doesn't cover all cases that were already implemented in previous years, so we'd need to transfer the tests.

Ideally we'd leverage the official TCK https://github.com/jakartaee/servlet/tree/master/tck to some extend.

Compare the cases covered through com.amazonaws.serverless.proxy.internal.testutils.AwsProxyRequestBuilder with com.amazonaws.serverless.proxy.spring.AwsSpringHttpProcessingUtilsTests. Start by running the Spring Cloud servlet-processing tests and identify implemented cases missing there. Transfer the relevant coverage, then assess whether the Jakarta Servlet TCK can add useful compliance checks; done means the new mapping path covers the prior cases and tests pass.

Written by the indexing model from the issue text.