fix(auth): prune stale custom model credentials

[tianma-if]

Summary

• prune stale custom endpoint credential-pool entries sourced from the previous model_config when a new custom endpoint is explicitly saved
• keep manual credentials for other custom providers intact
• add a regression test that drives the real custom model setup flow and verifies the old pool no longer keeps the replaced model_config credential

Fixes #57569.

Tests

• .venv/bin/python -m pytest tests/hermes_cli/test_custom_provider_model_switch.py -q
• .venv/bin/python -m ruff check hermes_cli/model_setup_flows.py tests/hermes_cli/test_custom_provider_model_switch.py
• git diff --check

[teknium1]

Merged via PR #58505 (#58505). Your commit was cherry-picked onto current main with your authorship preserved in git log (7915837). Thanks for the fix!