[cli] Fix rotate-key profile saving without an existing config

[plutohan]

Description

Fixes #9547. aptos key rotate-key --save-to-profile <name> failed with a config error when no config file (or default profile) existed, following the solution sketched in the issue:

1. The profile-name verification now only loads the config when one exists.
2. Saving starts from a fresh CliConfig when none exists; save() already creates the .aptos folder and config file.
3. The new profile no longer requires the current (default) profile; it falls back to an empty base profile when there is none, while still surfacing real config errors.

How Has This Been Tested?

cargo check -p aptos passes. Manually traced all three failure paths against CliConfig/ProfileOptions in aptos-cli-common.

Key Areas to Review

The ConfigNotFoundError match when building the new profile: other errors still propagate, only the missing-profile case falls back to defaults.

Type of Change

• Bug fix

Which Components or Systems Does This Change Impact?

• Aptos CLI/SDK

Checklist

• I have read and followed the CONTRIBUTING doc
• I have performed a self-review of my own code

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Note

Low Risk
Localized CLI post-rotation config path; no on-chain or auth logic changes, with explicit propagation of non-missing-config errors.

Overview
Fixes aptos key rotate-key --save-to-profile when there is no .aptos config or default profile yet.

Pre-rotation checks only load config to reject duplicate profile names when a config file already exists; missing config is allowed.

After a successful rotation, saving uses an existing config or CliConfig::default(), then config.save() creates the file. The new profile copies settings from the current profile when profile() succeeds, and uses ProfileConfig::default() only when the error is ConfigNotFoundError; other config errors still fail.

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