fix: normalize JSON output for wallet key commands

[GlobalStar117]

Summary

This PR normalizes JSON output for wallet key commands (regen_coldkey, regen_coldkey_pub, regen_hotkey) to provide a consistent structure across all operations.

Fixes #635

Changes

• Added a helper function _print_json_result(success, data, error) that standardizes JSON output format
• Refactored all wallet key commands to use this helper instead of inline json.dumps() calls
• Ensures consistent JSON structure: {"success": bool, "data": dict|null, "error": string|null}

Benefits

• Consistency: All wallet JSON outputs now follow the same structure
• Maintainability: Single source of truth for JSON formatting reduces code duplication
• Reliability: Eliminates potential inconsistencies between different command outputs

Testing

Tested manually with:

• btcli wallet regen-coldkey --json-output
• btcli wallet regen-coldkey-pub --json-output
• btcli wallet regen-hotkey --json-output

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This is a Gittensor contribution