fix: handle malformed mirror JSON responses

[bitloi]

Summary

Fix MirrorClient so malformed HTTP 2xx mirror responses are treated as mirror request failures instead of escaping as raw JSON parse exceptions.

Previously, _get() returned response.json() directly for any 2xx response. If the mirror, CDN, or proxy returned HTTP 200 with truncated JSON or an HTML/plaintext error body, ValueError could bypass retry handling and the MirrorRequestError path used by validator mirror callers.

This PR:

• retries invalid 2xx JSON with the existing mirror backoff policy
• raises MirrorRequestError after max_attempts
• wraps top-level mirror response schema parse failures as MirrorRequestError
• preserves existing validator fallback behavior for failed mirror fetches

Related Issues

Closes #893

Type of Change

• Bug fix
• New feature
• Refactor
• Documentation
• Other (describe below)

Testing

• Tests added/updated
• Manually tested

Commands run:

• uv run pytest tests/utils/test_mirror_client.py -q
• uv run pytest tests/validator/oss_contributions/mirror/test_load.py -q
• uv run pytest tests/validator/issue_discovery/test_mirror_scan.py::TestRunMirrorIssueDiscovery::test_mirror_request_error_does_not_abort_other_miners tests/validator/issue_discovery/test_mirror_scan.py::TestSolvingPrCache::test_fetch_failure_skips_scoring_for_that_issue -q
• uv run pytest tests/ -q --ignore=tests/validator/issue_discovery/test_one_issue_per_pr_cross_miner.py --ignore=tests/validator/issue_discovery/test_post_merge_issue_edit.py
• uv run pyright
• uv run pre-commit run --files gittensor/utils/mirror/client.py tests/utils/test_mirror_client.py tests/validator/oss_contributions/mirror/test_load.py

Checklist

• Code follows project style guidelines
• Self-review completed
• Changes are documented (if applicable)

[anderdc]

gittensor/utils/mirror/client.py:134 — use backoff_seconds(attempt) instead of inline min(5 * (2**attempt), 30). It's already imported and used by the other two retry tails in this same function; the inline copy drifts.

[bitloi]

gittensor/utils/mirror/client.py:134 — use backoff_seconds(attempt) instead of inline min(5 * (2**attempt), 30). It's already imported and used by the other two retry tails in this same function; the inline copy drifts.

@anderdc I just pushed the update. Would you please check it again?