fix(hermes): refresh model.base_url on provider switch

[lihan3238]

Summary

Fixes #2757. apply_switch_defaults in src-tauri/src/hermes_config.rs used ..current struct-update to preserve user customizations (context_length, max_tokens, extra). But this also silently preserved base_url across provider switches — so when the user switched from DeepSeek to Kimi For Coding, the model.base_url field in ~/.hermes/config.yaml kept pointing at DeepSeek. Auxiliary services (title generation, session search) then 404'd against the wrong endpoint.

The issue reporter (@KaisvenZ) provided a complete root-cause analysis including DB-vs-yaml diff evidence and pseudocode for the fix.

Change

src-tauri/src/hermes_config.rs apply_switch_defaults:

+    let new_base_url = settings_config
+        .get("base_url")
+        .and_then(|v| v.as_str())
+        .map(|s| s.trim().to_string())
+        .filter(|s| !s.is_empty());
+
     let current = get_model_config()?.unwrap_or_default();
     let merged = HermesModelConfig {
         default: first_model_id.or(current.default.clone()),
         provider: Some(provider_id.to_string()),
+        base_url: new_base_url.or(current.base_url.clone()),
         ..current
     };

Use the new provider's settings_config.base_url when set, falling back to the existing value when the new provider's settings omits base_url (so a user override survives a switch to a provider that does not declare its own).

Docstring updated to spell out the new precedence.

Tests

Added two regression tests:

• apply_switch_defaults_updates_base_url_to_new_provider — direct reproduction of [Bug] Hermes model.base_url not updated when switching providers (HTTP 404 on auxiliary services) #2757 (DeepSeek → Kimi switch with both providers declaring their own base_url)
• apply_switch_defaults_ignores_blank_base_url_in_settings — guards the fallback path so a whitespace-only base_url in settings does not blank out an existing override

The four pre-existing apply_switch_defaults tests continue to pass unchanged. None of them asserted base_url with a non-empty settings.base_url value — they covered:

• (sets_default_and_provider) settings had base_url but didn't assert on it → still passes; new behavior just makes the assertion possible
• (preserves_user_context_length) settings lacked base_url, asserts user override is preserved → still works (falls back to current via .or())
• (updates_provider_even_without_models) settings had base_url but didn't assert on it → still passes
• (keeps_old_default_when_first_model_id_is_blank) settings lacked base_url → still works

Behavior change note

This change refreshes model.base_url whenever the new provider declares one, which is a user-visible behavior change. The previous code would have kept a manually-edited model.base_url even across a provider switch. Two views are reasonable:

• Issue reporter's view (this PR): provider switching is a "use this provider's settings" gesture; provider-specific fields including base_url should refresh.
• Alternative view: user-set overrides should always win, even across provider switches.

Happy to defer if you prefer the alternative — the simplest revert is to drop the base_url: line in the struct literal and remove the new test. I went with the issue-reporter's view because the breakage they describe (404s on auxiliary services) is consistent and affects real users, and because the previous "always preserve" behavior was implicit in the ..current rather than explicitly designed.

Verification

• cargo fmt --check passes locally
• Could not run cargo test locally (Tauri build needs pkg-config + libssl-dev system packages I don't have sudo for); diff is small and identical in shape to surrounding tested code, relying on the backend CI job to verify.

[farion1231]

@codex review

[chatgpt-codex-connector]

Codex Review: Didn't find any major issues. Bravo.

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