fix: clear stale api keys when switching API-key providers

[LeonSGP43]

Summary

• clear stale inline model.api_key values when switching API-key providers in hermes model
• keep provider/base URL updates while removing leftover credentials from the previous provider
• add a regression test covering a switch from OpenRouter to a generic API-key provider

Testing

• python3 -m pytest -o addopts= tests/hermes_cli/test_model_provider_persistence.py

Closes #14134.

[alt-glitch]

Likely duplicate of #14370 — same root cause (stale api_key on provider switch in hermes_cli/main.py), same target issue #14134. Also related to #8571.

[teknium1]

Closing — issue #14134 is resolved on current main, so this PR is no longer needed. Thanks for the contribution and analysis.