fix(cli): report vote list validator read failures as read_failed

[carlos4s]

Summary

Fixes #1471

Fix gitt vote list --json so validator whitelist read failures no longer look like a successful empty whitelist.

Changes:

• Add ContractReadError for trustworthy contract-read failure reporting.
• Make IssueCompetitionContractClient.get_validators() raise on failed raw reads, read exceptions, and malformed validator Vec payloads.
• Preserve successful empty whitelist behavior for a valid encoded empty Vec.
• Map validator whitelist read failures in vote list to error.type == "read_failed".
• Add CLI regression tests for raw read None, read exception, malformed payload, and real empty whitelist.

Verification (live finney — real data, no mocks)

Run against the live finney network using the exact contract addresses from #1471:

Contract	Meaning
5DU266Ac3kVL6b3gGKNVqQ467WAq4JpucRAwxU1z9B98Bkav	deployed non-issues contract — the get_validators read fails
5FWNdk8YNtNcHKrAx2krqenFrFAZG7vmsd2XN2isJSew3MrD	real issues contract — returns the live validator whitelist

Before / After — failed read

Same failed-read contract, same command. Before this PR a failed read was reported as a successful empty whitelist (success: true, exit 0); after it is correctly surfaced as read_failed (exit 1).

gitt vote list --json --network finney \
  --contract 5DU266Ac3kVL6b3gGKNVqQ467WAq4JpucRAwxU1z9B98Bkav

Regression check — real whitelist still reads correctly

A genuine read against the real issues contract is unchanged: it still succeeds and returns the live validator (success: true, exit 0).

gitt vote list --json --network finney \
  --contract 5FWNdk8YNtNcHKrAx2krqenFrFAZG7vmsd2XN2isJSew3MrD

Testing

• uv run --extra dev pytest
• uv run --extra dev ruff check .
• uv run --extra dev ruff format --check .
• uv run --extra dev pyright

[anderdc]

please provide screenshots of the cli command working

[carlos4s]

@anderdc I've attached cli command working screenshots in PR description. Please review again.