Return 400 when canonical_alias content invalid (#19240)

Fixes #19198

Returns HTTP 400 when `alias` or `alt_alias` inside of
`m.room.canonical_alias` `content` are not of type string.
Previously this resulted in HTTP 500 errors as Synapse assumed they were
strings and would raise an exception when it tried to treat them as such
if they actually weren't.

With the changes implemented:
<img width="800" height="616" alt="Screenshot from 2025-11-28 16-48-06"
src="https://github.com/user-attachments/assets/1333a4b3-7b4f-435f-bbff-f48870bc4d96"
/>
<img width="800" height="316" alt="Screenshot from 2025-11-28 16-47-42"
src="https://github.com/user-attachments/assets/5928abf8-88a2-4bd9-9420-9a1f743f66f5"
/>

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changelog.d/19240.bugfix

@@ -0,0 +1 @@
+Fix bug where invalid `canonical_alias` content would return 500 instead of 400.