fix: call bind (not set_active) when switching to a fresh session

Bug: 400 from PUT /opencode-bridge/sessions/active when the user picked
a session from the global list that wasn't yet bound to this notebook.
The cell-action onSwitchSession callback was:
  1. PUT /sessions/active (server checks bound -> 400)
  2. PUT /sessions/notebook (bind, but never reached)
The order was wrong: set_active requires the session to be bound FIRST.

Fix: the PUT /sessions/notebook route does both bind AND set active
in a single call (server handler: bind_existing() then response
includes the new activeSessionId). So the callback now only needs to
call bind-existing. The /sessions/active endpoint is still useful for
future "switch between already-bound sessions" flows but the current
"pick from global list" path doesn't need it.

Tests: updated the switching test to assert bind-existing is called
and set-active is NOT. Also added not.toHaveBeenCalled() to lock in
the new behavior. 76 jest, 73 pytest, build green.

Co-Authored-By: Claude <noreply@anthropic.com>

src/__tests__/opencode_cell_actions.spec.ts

@@ -1123,9 +1123,6 @@ describe('OpenCodeInlinePrompt', () => {
         { id: 'sess-B', title: 'second' },
       ]
     });
-    mockedSetActiveSession.mockResolvedValue({
-      ok: true, notebookPath: 'foo.ipynb', activeSessionId: 'sess-B',
-    });
     mockedBindExistingSession.mockResolvedValue({
       ok: true, notebookPath: 'foo.ipynb', sessionId: 'sess-B',
     });
@@ -1144,7 +1141,11 @@ describe('OpenCodeInlinePrompt', () => {
     select.value = 'sess-B';
     select.dispatchEvent(new Event('change'));
     await new Promise(r => setTimeout(r, 5));
-    expect(mockedSetActiveSession).toHaveBeenCalled();
+    // bind-existing is the single call (it does both bind AND set
+    // active on the server). Calling set_active first would 400
+    // because the session is not yet bound.
+    expect(mockedBindExistingSession).toHaveBeenCalled();
+    expect(mockedSetActiveSession).not.toHaveBeenCalled();
     expect((prompt as any)._sessionId).toBe('sess-B');
   });