fix(python-sdk): route Myriad wallet address correctly

[realfishsam]

Summary

• Fixes Myriad(wallet_address=...) in the Python SDK so the address populates the base wallet_address slot instead of private_key.
• Adds a regression test proving the public wallet address no longer triggers private-key signer setup or leaves wallet_address unset.

Fixes #1448

Test Plan

• uv run --with pytest --with httpx pytest sdks/python/tests/test_hosted_dispatch.py::test_myriad_wallet_address_populates_wallet_slot_not_private_key -q
• uv run --with pytest --with httpx pytest sdks/python/tests/test_hosted_dispatch.py -q

Note: local checkout lacks committed generated pmxt_internal artifacts, so I used an untracked temporary generated-client stub only for local test importability and removed it before staging.