refactor: add is_already_voted predicate for AlreadyVoted rejections

[dev-miro26]

Closes: #100

Summary

• Add is_already_voted(e) alongside is_contract_rejection in allways/contract_client.py — a single source of truth for detecting the AlreadyVoted contract-error variant, anchored to CONTRACT_ERROR_VARIANTS[3].
• Replace two inline 'AlreadyVoted' in str(e) substring checks in allways/validator/forward.py with calls to the new predicate.
• No behavior change — the detection semantics are identical today; this is purely a centralization so the substring literal lives in exactly one place.

Why

forward.py:254 and forward.py:395 were branching on if 'AlreadyVoted' in str(e):. The codebase already exports a structured CONTRACT_ERROR_VARIANTS map and a canonical is_contract_rejection(e) predicate from allways/contract_client.py, but the AlreadyVoted check bypassed both with a raw substring match. If the ContractError message format ever changes (e.g. variant rename, localisation, format tweak in decode_contract_error), the two call sites would silently stop matching with no lint or test signal.

Co-locating the predicate with is_contract_rejection also leaves room for a small documented predicate set (is_duplicate_source_tx, is_miner_reserved, …) if future code wants variant-specific branching.

Changes

• allways/contract_client.py: add is_already_voted(e) -> bool next to is_contract_rejection; docstring points at CONTRACT_ERROR_VARIANTS[3] as the canonical variant source.
• allways/validator/forward.py:
  • Import is_already_voted alongside the existing is_contract_rejection.
  • forward.py:254 (extend-reservation vote failure branch) — swap substring check for predicate.
  • forward.py:395 (extend-timeout vote failure branch) — swap substring check for predicate.
• Net +8 / −2 lines; no signature changes.

Type of Change

• Refactoring (centralisation of an existing predicate, no behavior change)

Test plan

• ruff check allways/ neurons/ tests/ — clean
• ruff format --check allways/ neurons/ tests/ — clean
• pytest tests/ — 291 passed; 9 pre-existing failures in tests/test_axon_handlers.py on upstream/test are unrelated ("too many values to unpack (expected 3)" in the SwapConfirm handler) and unchanged by this diff
• rg "'AlreadyVoted' in str" — now returns exactly one hit (the predicate itself), zero remaining inline substring checks

[dev-miro26]

Hi, @LandynDev
Could you please review my first PR?
Thank you.

[anderdc]

Closing — is_already_voted(e) has a single call site (try_extend_reservation) and replaces 'AlreadyVoted' in str(e) (one line) with is_already_voted(e) (one line). No call-site simplification, just an indirection. The neighbor is_contract_rejection and other error-string checks throughout contract_client.py still use raw in str(e) patterns — extracting one of them in isolation is churn rather than cleanup.