refactor: extract duplicate network/contract resolution to _resolve_contract_and_network helper

[bittoby]

Summary

Every issue CLI command started with the same 5-line block - get_contract_address, resolve_network, then an if not contract_addr guard. This was copy-pasted across 15 call sites in 4 files. Extracted it into _resolve_contract_and_network() in helpers.py

What changed

• Added _resolve_contract_and_network(contract, network, rpc_url) to helpers.py. Returns (contract_addr, ws_endpoint, network_name), raises click.ClickException if contract is empty. Accepts an optional missing_contract_message for the 3 commands that had custom error text.
• Replaced all 15 duplicated blocks:
  • admin.py - 6 commands
  • view.py - 4 commands (1 with custom error message)
  • vote.py - 3 commands
  • mutations.py - 2 commands (both with custom error messages)
• submissions.py left unchanged - it uses handle_exception(as_json, ...) instead of raise, so the shared helper doesn't fit.
• Updated test patches to mock _resolve_contract_and_network instead of the removed module-level imports.

Testing

• Ruff lint: clean
• Ruff format: clean
• Pyright: 0 errors, 0 warnings
• Pytest: 300 passed, 0 failed

[bittoby]

@anderdc Pls review this PR.