feat(ssh): pre-populate vault password in auth prompts for MFA (fixes #9911) (#11170)

* feat(ssh): pre-populate vault password in auth prompts for MFA use cases

When a keyboard-interactive password prompt is shown and the response
field is empty, load and pre-fill the saved vault password. This lets
users with hardware tokens (e.g. YubiKey) press their token to append
a TOTP to the pre-filled base password without re-typing it.

Also pre-fill the prompt-password modal with the vault password so the
same workflow applies when SSH password auth is used with MFA.

Closes #9911

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

* fix(ssh): remove unnecessary null coalescing in loadPassword call

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

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Co-authored-by: Claude Sonnet 4.6 <noreply@anthropic.com>