fix(proxy): wire /config/update into the new config DualCache invalidation

After merging litellm_internal_staging (which introduced
litellm_config_cache and `await invalidate_config_param(...)` calls
paired with every config write), the rewritten /config/update was left
in a broken state: the auto-merge stranded one `await
invalidate_config_param(k)` call inside the new litellm_settings block
where `k` is undefined, raising UnboundLocalError on every request that
included litellm_settings.

Bake invalidation into the local `_upsert_section` helper so each
section write atomically invalidates its own cache key — there's no
longer a per-section call site to remember to update. Drop the stray
`invalidate_config_param(k)` line.

This restores tests/proxy_unit_tests/test_proxy_server.py::
test_update_config_success_callback_normalization, which was the only
failing test on the proxy-server GHA shard.

litellm/proxy/proxy_server.py

@@ -12698,6 +12698,9 @@ async def update_config(  # noqa: PLR0915
                     "update": {"param_value": serialized},
                 },
             )
+            # invalidate the DualCache entry so the next reader (this process
+            # or any other proxy in the cluster) goes to DB.
+            await invalidate_config_param(param_name)
 
         # general_settings: merge per-key, with the alert_to_webhook_url side
         # effect of auto-enabling slack alerting.
@@ -12740,7 +12743,6 @@ async def update_config(  # noqa: PLR0915
                 updated_litellm_settings["success_callback"] = normalize_callback_names(
                     incoming_cb
                 )
-                await invalidate_config_param(k)
 
             merged = {**existing, **updated_litellm_settings}