fix(setup): write config for image_gen and video_gen in apply_nous_managed_defaults (#35109)

apply_nous_managed_defaults() was adding image_gen and video_gen to the
'changed' return set without writing any config values.  The caller
(tools_command first_install flow) uses 'changed' to skip manual
configuration, so these tools ended up in platform_toolsets but with no
video_gen.provider, video_gen.use_gateway, or image_gen.use_gateway in
config.yaml.

At runtime the FAL plugin's is_available() returned False because there
was no FAL_KEY and no use_gateway config — the tool never loaded despite
being 'enabled' in the toolset list.

For image_gen this was a latent bug masked by the gateway offer prompt
(prompt_enable_tool_gateway) running earlier in the setup flow and
writing image_gen.use_gateway=True via apply_gateway_defaults().  But if
the user skipped the gateway offer, image_gen would silently break the
same way.

For video_gen (added in PR #33259) the bug was always hit because the
gateway offer ran before the user checked video_gen in the toolset
checklist.

Fix: write provider/use_gateway config values before adding to 'changed',
matching the pattern used by web, tts, and browser.

tests/honcho_plugin/test_async_memory.py

@@ -249,9 +249,12 @@ class TestFlushAll:
         mgr = _make_manager(write_frequency="async")
         sess = _make_session()
         sess.add_message("user", "pending")
-        mgr._async_queue.put(sess)
 
         with patch.object(mgr, "_flush_session") as mock_flush:
+            # Put the item AFTER the mock is installed so the background
+            # writer thread (if it dequeues before flush_all) still hits
+            # the mock rather than the real _flush_session.
+            mgr._async_queue.put(sess)
             mgr.flush_all()
             # Called at least once for the queued item
             assert mock_flush.call_count >= 1