Don't assert human error message in TestAsyncUpload (de-flake) (#839)

As a general rule, we should not be asserting the language of human `error` messages. They can change at any time, localized, etc.

This is spawning from a real life flaky failure I'm seeing when testing with the workers variant of Synapse:

```shell
$ WORKERS=1 COMPLEMENT_DIR=../complement ./scripts-dev/complement.sh ./tests/csapi -run TestAsyncUpload/Cannot_upload_to_a_media_ID_that_has_already_been_uploaded_to
...
=== NAME  TestAsyncUpload/Cannot_upload_to_a_media_ID_that_has_already_been_uploaded_to
    media_async_uploads_test.go:68: MatchResponse key 'error' got 'Media ID cannot be overwritten' (type string) want 'Media ID already has content' (type string) - http://127.0.0.1:33651/_matrix/media/v3/upload/hs1/GjRDDFucigeiLQtzNbjADIoI => {"errcode":"M_CANNOT_OVERWRITE_MEDIA","error":"Media ID cannot be overwritten"}
2026/01/22 17:05:44 ============================================
    
--- FAIL: TestAsyncUpload (14.13s)
    --- FAIL: TestAsyncUpload/Cannot_upload_to_a_media_ID_that_has_already_been_uploaded_to (0.07s)
FAIL
FAIL    github.com/matrix-org/complement/tests/csapi    16.063s
FAIL
```

I also see this test listed in the known flakes we track with Synapse: element-hq/synapse#18537

This problem happens because there are two possible error messages in Synapse which we could encounter for `M_CANNOT_OVERWRITE_MEDIA`:

 - [`"Media ID already has content"`](https://github.com/element-hq/synapse/blob/826a7dd29aea7041fb25b98d360da4dbedf22712/synapse/media/media_repository.py#L304-L308)
 - [`"Media ID cannot be overwritten"`](https://github.com/element-hq/synapse/blob/826a7dd29aea7041fb25b98d360da4dbedf22712/synapse/rest/media/upload_resource.py#L159-L163)

Author: MadLittleMods

tests/csapi/media_async_uploads_test.go

@@ -42,7 +42,7 @@ func TestAsyncUpload(t *testing.T) {
 			StatusCode: http.StatusGatewayTimeout,
 			JSON: []match.JSON{
 				match.JSONKeyEqual("errcode", "M_NOT_YET_UPLOADED"),
-				match.JSONKeyEqual("error", "Media has not been uploaded yet"),
+				match.JSONKeyPresent("error"),
 			},
 		})
 	})
@@ -64,7 +64,7 @@ func TestAsyncUpload(t *testing.T) {
 			StatusCode: http.StatusConflict,
 			JSON: []match.JSON{
 				match.JSONKeyEqual("errcode", "M_CANNOT_OVERWRITE_MEDIA"),
-				match.JSONKeyEqual("error", "Media ID already has content"),
+				match.JSONKeyPresent("error"),
 			},
 		})
 	})