refactor: inline single-use _get_child_storage_key method

[ebios-star]

Summary

`_get_child_storage_key` on `IssueCompetitionContractClient` (gittensor/validator/issue_competitions/contract_client.py:127-133) was a 7-line private method called from exactly one place: `read_issue_from_child_storage`. Same single-use-helper pattern as the merged #748, #801, and #802.

The inlined try/except is preserved verbatim and kept distinct from the existing decode-side try/except in the caller, so the per-stage debug log messages ("Error getting child storage key" vs "Error reading issue {id}") stay attributable to the right phase.

`grep -rn _get_child_storage_key` confirms no other call sites in `gittensor/`, `neurons/`, or `tests/`.

Net: -9 / +5 lines, one fewer private method on the contract client. No behavior change.

Related Issues

N/A — same family of cleanups as #641 / #748 / #801 / #802.

Type of Change

• Bug fix
• New feature
• Refactor
• Documentation
• Other

Testing

• `grep -rn _get_child_storage_key` confirms a single call site.
• `uv run --extra dev pytest tests/` — all 652 tests pass.
• `tests/utils/test_issue_competitions_storage_utils.py` covers the underlying `get_contract_child_storage_key` and still passes.

Checklist

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

[ebios-star]

@anderdc @LandynDev — friendly ping for review when you have a moment.

Same family as #748 / #801 / #802 (all merged) — single-use private method inlined into its only caller. _get_child_storage_key on IssueCompetitionContractClient was a 7-line method called from exactly one place (read_issue_from_child_storage). The inlined try/except is preserved verbatim and kept distinct from the existing decode-side try/except so the per-stage debug logs stay attributable. grep confirms a single call site, no test references. -9/+5 lines, full test suite (652) passes.

Happy to rebase if needed.