fix: polish wg codec errors and validation (fixes #130)

[acul71]

Summary

• Align wg validation with certhash (validate() from to_bytes / to_string).
• Clearer errors for pasted wg(8) / std-base64 keys; fixed-byte negative tests.
• Add examples/wg/wg_examples.py and docs wiring.
• Fixes Follow-up: polish wg codec after #108 (errors, tests, certhash consistency) #130.

Demo

=== Valid WireGuard multiaddr ===
Input:  /ip4/1.2.3.4/udp/51820/wg/uAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA
...
=== Pasted std-base64 (wg(8) style) is rejected with a conversion hint ===

Test plan

• pytest tests
• newsfragment 130.bugfix.rst
• python examples/wg/wg_examples.py

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