Summary

Fixes #13420 — it is currently impossible to persist an empty llm_base_url from the settings UI.

Root cause: store_llm_settings used if not settings.llm_base_url: which catches both None (field not provided at all) and "" (explicitly cleared by the user). When the user:

• saves from Basic view (which intentionally sends llm_base_url: "" to clear advanced settings), or
• deletes the Base URL field in Advanced view and saves,

the backend treated "" the same as "not provided", triggering auto-detection via get_provider_api_base() and overwriting the user's intent. The old base URL kept coming back.

Fix: Split the condition into two explicit branches:

# Before
if not settings.llm_base_url:
    if settings.llm_base_url is None and existing_settings.llm_base_url:
        ...  # preserve existing

# After
if settings.llm_base_url is None:
    ...  # not provided — preserve existing or auto-detect (no behavior change)
elif settings.llm_base_url == "":
    settings.llm_base_url = None  # explicit clear — honor the intent

The None path has identical behavior to before. Only "" changes: instead of triggering auto-detection, it now normalizes to None and returns.

Test plan

• Updated test_store_llm_settings_partial_update to assert None (the correct post-fix behavior) instead of the auto-detected URL
• Added test_store_llm_settings_advanced_view_clear_removes_base_url as a named regression test for this issue
• All 16 tests in test_settings_store_functions.py pass
• Existing MCP save test (test_store_llm_settings_mcp_update_preserves_base_url) continues to pass — the None path (field not sent) is unchanged

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