Respond with useful error codes when Content-Length header/s are invalid (#19212)

Related to https://github.com/element-hq/synapse/issues/17035, when
Synapse receives a request that is larger than the maximum size allowed,
it aborts the connection without ever sending back a HTTP response.
I dug into our usage of twisted and how best to try and report such an
error and this is what I came up with.

It would be ideal to be able to report the status from within
`handleContentChunk` but that is called too early on in the twisted http
handling code, before things have been setup enough to be able to
properly write a response.
I tested this change out locally (both with C-S and S-S apis) and they
do receive a 413 response now in addition to the connection being
closed.

Hopefully this will aid in being able to quickly detect when
https://github.com/element-hq/synapse/issues/17035 is occurring as the
current situation makes it very hard to narrow things down to that
specific issue without making a lot of assumptions.

This PR also responds with more meaningful error codes now in the case
of:
- multiple `Content-Length` headers
- invalid `Content-Length` header value
- request content size being larger than the `Content-Length` value

### Pull Request Checklist

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---------

Co-authored-by: Eric Eastwood <erice@element.io>

tests/test_server.py

@@ -212,6 +212,66 @@ class JsonResourceTests(unittest.TestCase):
         self.assertEqual(channel.code, 200)
         self.assertNotIn("body", channel.result)
 
+    def test_content_larger_than_content_length(self) -> None:
+        """
+        HTTP requests with content size exceeding Content-Length should be rejected with 400.
+        """
+
+        def _callback(
+            request: SynapseRequest, **kwargs: object
+        ) -> tuple[int, JsonDict]:
+            return 200, {}
+
+        res = JsonResource(self.homeserver)
+        res.register_paths(
+            "POST", [re.compile("^/_matrix/foo$")], _callback, "test_servlet"
+        )
+
+        channel = make_request(
+            self.reactor,
+            FakeSite(res, self.reactor),
+            b"POST",
+            b"/_matrix/foo",
+            {},
+            # Set the `Content-Length` value to be smaller than the actual content size
+            custom_headers=[("Content-Length", "1")],
+            # The request should disconnect early so don't await the result
+            await_result=False,
+        )
+
+        self.reactor.advance(0.1)
+        self.assertEqual(channel.code, 400)
+
+    def test_content_smaller_than_content_length(self) -> None:
+        """
+        HTTP requests with content size smaller than Content-Length should be rejected with 400.
+        """
+
+        def _callback(
+            request: SynapseRequest, **kwargs: object
+        ) -> tuple[int, JsonDict]:
+            return 200, {}
+
+        res = JsonResource(self.homeserver)
+        res.register_paths(
+            "POST", [re.compile("^/_matrix/foo$")], _callback, "test_servlet"
+        )
+
+        channel = make_request(
+            self.reactor,
+            FakeSite(res, self.reactor),
+            b"POST",
+            b"/_matrix/foo",
+            {},
+            # Set the `Content-Length` value to be larger than the actual content size
+            custom_headers=[("Content-Length", "10")],
+            # The request should disconnect early so don't await the result
+            await_result=False,
+        )
+
+        self.reactor.advance(0.1)
+        self.assertEqual(channel.code, 400)
+
 
 class OptionsResourceTests(unittest.TestCase):
     def setUp(self) -> None: