Refactor event stream tests with `{client,server}IntegrationTest`s (#2342)

* Refactor `ClientEventStreamUnmarshallerGeneratorTest` to use `clientIntegrationTest` (WIP)

* Refactor `ClientEventStreamUnmarshallerGeneratorTest` with `clientIntegrationTest`

* Refactor `ClientEventStreamUnmarshallerGeneratorTest` to use generic test cases

* Start refactoring `ServerEventStreamUnmarshallerGeneratorTest`

* Make `ServerEventStreamUnmarshallerGeneratorTest` tests work

* Uncomment other test models

* Allow unused on `parse_generic_error`

* Rename `ServerEventStreamUnmarshallerGeneratorTest`

* Make `EventStreamUnmarshallTestCases` codegenTarget-agnostic

* Refactor `ClientEventStreamMarshallerGeneratorTest`: Tests run but fail

* Refactor `ServerEventStreamMarshallerGeneratorTest`

* Move `.into()` calls to `conditionalBuilderInput`

* Add "context" to TODO

* Fix client unmarshall tests

* Fix clippy lint

* Fix more clippy lints

* Add docs for `event_stream_serde` module

* Fix client tests

* Remove `#[allow(missing_docs)]` from event stream module

* Remove unused `EventStreamTestTools`

* Add `smithy-validation-model` test dep to `codegen-client`

* Temporarily add docs to make tests compile

* Undo change in model

* Make event stream unmarshaller tests a unit test

* Remove unused code

* Make `ServerEventStreamUnmarshallerGeneratorTest` a unit test

* Make `ServerEventStreamMarshallerGeneratorTest` a unit test

* Make `ServerEventStreamMarshallerGeneratorTest` pass

* Make remaining tests non-integration tests

* Make event stream serde module private again

* Remove unnecessary clippy allowances

* Remove clippy allowance

* Remove docs for `event_stream_serde` module

* Remove docs for `$unmarshallerTypeName::new`

* Remove more unnecessary docs

* Remove more superfluous docs

* Undo unnecessary diffs

* Uncomment last test

* Make `conditionalBuilderInput` internal