fix(sessions): copy modelProvider on session reset to prevent stale provider cross-contamination

When a session is reset (via /new or /reset), the previous entry's model
field was copied to the new entry but modelProvider was not. This left the
new entry in a state where resolveSessionModelRef would fall back to the
config default_model provider (e.g. google-gemini-cli) when the stored
model string has no provider prefix — producing a wrong display like
'google-gemini-cli/claude-sonnet-4-6' in the TUI status bar.

Fixes the root cause for issue #25408 alongside the defensive fix in
session-utils.ts.

src/gateway/server-methods/sessions.ts

@@ -387,6 +387,7 @@ export const sessionsHandlers: GatewayRequestHandlers = {
         reasoningLevel: entry?.reasoningLevel,
         responseUsage: entry?.responseUsage,
         model: entry?.model,
+        modelProvider: entry?.modelProvider,
         contextTokens: entry?.contextTokens,
         sendPolicy: entry?.sendPolicy,
         label: entry?.label,