Use the validator admin API as the only private recovery interface

[#2393](https://github.com/0xMiden/node/pull/2393) added local commands to export a private record and issue its Golden decryption share. [#2407](https://github.com/0xMiden/node/pull/2407) adds the same recovery flow to a private admin API. Keeping both creates two places for access rules and tests.

Keep the admin API. The recovery UI already needs it, and it can issue a share for another validator's record. After #2407 lands, remove `export-private-record` and `issue-private-record-share`. Keep the shared Golden helpers used by the admin service, and move any needed recovery tests to that service.

Blocked by #2407.