refactor(validator): narrow _sign_with_key return to Signature

[tcoratger]

Summary

The signing helper in the validator service returned a tuple[ValidatorEntry, Signature], but the updated entry was never used by any caller. Both call sites discarded it with _,, and the registry is already updated in place inside the helper before it returns. The wider return type was dead surface area.

This narrows the return to Signature:

• Drop the unused tuple element from the helper; return the signature directly.
• Update both call sites to bind the signature without the discard placeholder.
• Update the docstring Returns: section accordingly.
• Simplify the unit test that previously unpacked the tuple — registry persistence is already covered by a dedicated test.

The type narrowing is validated by ty; just check passes clean.

Testing

• uv run pytest tests/node/validator/test_service.py -q — 51 passed.
• just check — lint, format, typecheck, spell, mdformat all pass.

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