Ensure we /send PDUs which pass canonical JSON checks (#18641)

### Pull Request Checklist

Fixes https://github.com/element-hq/synapse/issues/18554

Looks like this was missed when it was
[implemented](2277df2a1e).

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* [x] Pull request is based on the develop branch
* [x] Pull request includes a [changelog
file](https://element-hq.github.io/synapse/latest/development/contributing_guide.html#changelog).
The entry should:
- Be a short description of your change which makes sense to users.
"Fixed a bug that prevented receiving messages from other servers."
instead of "Moved X method from `EventStore` to `EventWorkerStore`.".
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* [x] [Code
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correct (run the
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---------

Co-authored-by: reivilibre <oliverw@element.io>

changelog.d/18641.bugfix

@@ -0,0 +1 @@
+Ensure all PDUs sent via `/send` pass canonical JSON checks.

synapse/federation/sender/transaction_manager.py

@@ -26,7 +26,7 @@ from synapse.api.constants import EduTypes
 from synapse.api.errors import HttpResponseException
 from synapse.events import EventBase
 from synapse.federation.persistence import TransactionActions
-from synapse.federation.units import Edu, Transaction
+from synapse.federation.units import Edu, Transaction, serialize_and_filter_pdus
 from synapse.logging.opentracing import (
     extract_text_map,
     set_tag,
@@ -119,7 +119,7 @@ class TransactionManager:
                 transaction_id=txn_id,
                 origin=self.server_name,
                 destination=destination,
-                pdus=[p.get_pdu_json() for p in pdus],
+                pdus=serialize_and_filter_pdus(pdus),
                 edus=[edu.get_dict() for edu in edus],
             )