fix(bounties): reject ambiguous solver identities

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Summary

• Exclude failed miner evaluations from issue-bounty solver lookup.
• Detect GitHub IDs that map to multiple active miner hotkeys and cancel matching bounty votes instead of selecting one by dictionary overwrite.
• Preserve bounty payout behavior for non-eligible but otherwise valid registered miners.

What changed

• Added a safe GitHub ID to hotkey lookup builder for issue bounty voting.
• Added an explicit ambiguous solver cancel path before coldkey lookup and solution voting.
• Added regression tests for failed miners, ambiguous GitHub IDs, and the existing non-eligibility-gated bounty behavior.

Why

Bounty solver lookup should only vote for a solver when the validator has exactly one non-failed registered miner identity for the solver's GitHub account. Failed or ambiguous current-round identities are not reliable payout targets.

Validation

• uv run pytest tests/validator/test_issue_competitions_forward.py tests/validator/test_duplicate_penalty_propagation.py -q
• uv run ruff check gittensor/validator/issue_competitions/forward.py tests/validator/test_issue_competitions_forward.py
• uv run ruff format --check gittensor/validator/issue_competitions/forward.py tests/validator/test_issue_competitions_forward.py
• uv run pyright gittensor/validator/issue_competitions/forward.py tests/validator/test_issue_competitions_forward.py
• git diff --check
• Codex Security source-to-sink review: failed and ambiguous identities cannot reach coldkey lookup or solution voting; non-eligible but otherwise valid registered miners still can.
• CodeRabbit CLI review: 0 issues.