dns_yc: fix key handling and unify add/rm key preparation

[zhuravlev-nik]

Previously:

• YC_SA_Key_File was removed unconditionally after signing,
  which deleted user-provided key files when using
  YC_SA_Key_File_Path.
• dns_yc_rm() did not prepare the key file the same way as
  dns_yc_add(), which could lead to inconsistent behavior
  depending on call order.

Changes:

• Do not delete persistent YC_SA_Key_File_Path.
• Prepare key material consistently in both add() and rm().
• Use internal _mktemp() for temporary key files created
  from YC_SA_Key_File_PEM_b64.
• Clean up temporary key files via POSIX-compatible trap.

The change is backward compatible and preserves existing
configuration behavior.

[github-actions]

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