jid: Fix JID parts serde deserialization

serde was bypassing the validation these types rely on, making them
completely unsound in an XMPP context.

Fixes #165.

jid/CHANGELOG.md

@@ -2,6 +2,9 @@ Version NEXT:
 
 Version 0.12.1, release 2025-11-02:
   * Changes:
+    - Make serde deserialization actually validate the `NodePart`, `DomainPart`
+      and `ResourcePart`, it was previously completely unsound, creating these
+      types without validating their invariants.
     - The 'quote' feature now uses `jid::Jid` instead of `::jid::Jid` to
       stop requiring importing the module as a dependency of the project. The
       `jid` module just needs to be made available, for example: `use