fix(desktop): preserve local model provider endpoint

[AtakanGs]

Fixes #65201

Summary

Preserves the configured endpoint when selecting a local or user-defined model provider in the Desktop model settings.

Previously, Desktop displayed the selected Ollama provider and model correctly, but the model assignment request omitted the provider endpoint. The backend then cleared the previous base_url, leaving the local provider without an endpoint and allowing runtime resolution to fall back to OpenRouter-compatible handling.

Root cause

/api/model/options exposes the endpoint for user-defined providers as api_url.

The Desktop ModelOptionProvider type did not include that field, and applyMainModel() only submitted:

• model
• provider
• scope

As a result, switching to Ollama/custom persisted the provider selection without persisting its endpoint.

Fix

• Add api_url to ModelOptionProvider.
• Pass the selected provider's api_url back as base_url when applying the main model.
• Preserve existing behavior for providers that do not expose a custom endpoint.
• Add a regression test covering a user-defined Ollama provider.

Testing

• Added regression test:
  preserves a user-defined provider endpoint when applying the main model
• Focused Desktop test suite: 11 passed
• ESLint on changed files: 0 errors
  • One existing react-hooks/exhaustive-deps warning remains outside this change.
• git diff --check: passed
• Rebased onto the latest origin/main and reran the focused tests successfully.

Typecheck note

The full Desktop typecheck is currently blocked by 7 unrelated errors in existing assistant-ui integration files involving installed package API mismatches. None of the reported errors are in the files changed by this PR.

[teknium1]

Thanks for the focused regression fix. Current main still posts the settings selection without base_url at apps/desktop/src/app/settings/model-settings.tsx:589, while the backend clears a prior endpoint on a provider change without an explicit replacement at hermes_cli/web_server.py:1161-1167. The picker contract exposes configured-provider endpoints as api_url at hermes_cli/model_switch.py:2381-2390; forwarding that value as this PR does is consistent with the assignment API.

The added Ollama regression assertion covers the reported settings-path failure.

Automated hermes-sweeper review.