fix(gateway): clear auto-fallback model override on session reset (#63155)

* fix(gateway): clear auto-fallback model override on session reset

When `persistFallbackCandidateSelection()` writes a fallback provider
override with `authProfileOverrideSource: "auto"`, the override was
incorrectly preserved across `/reset` and `/new` commands. This caused
sessions to keep using the fallback provider even after the user changed
the agent config primary provider, because the session store override
takes precedence over the config default.

Now the override fields (`providerOverride`, `modelOverride`,
`authProfileOverride`, `authProfileOverrideSource`,
`authProfileOverrideCompactionCount`) are only carried forward when
`authProfileOverrideSource === "user"` (i.e. explicit `/model` command).
System-driven overrides are dropped on reset so the session picks up the
current config default.

Introduced in cb0a752156 ("fix: preserve reset session behavior config")

* fix(gateway): preserve explicit reset model selection

* fix(gateway): track reset model override source

* fix(gateway): preserve legacy reset model overrides

* docs(changelog): add session reset merge note

---------

Co-authored-by: termtek <termtek@ubuntu.tail2b72cd.ts.net>

src/config/sessions/types.ts

@@ -162,6 +162,12 @@ export type SessionEntry = {
   responseUsage?: "on" | "off" | "tokens" | "full";
   providerOverride?: string;
   modelOverride?: string;
+  /**
+   * Tracks whether the persisted model override came from an explicit user
+   * action (`/model`, `sessions.patch`) or from a temporary runtime fallback.
+   * Resets only preserve user-driven overrides.
+   */
+  modelOverrideSource?: "auto" | "user";
   authProfileOverride?: string;
   authProfileOverrideSource?: "auto" | "user";
   authProfileOverrideCompactionCount?: number;