fix: createTask with undefined input values fails to create task

Bug: Setting a task input property to undefined (e.g. { prune: undefined })
to express "this key should be deleted" resulted in no task being created.
JSON.stringify strips undefined values, so { prune: undefined } serialized
as {}, and is_partial_of({}, any_config) always returns true — meaning
input-not-matches saw a "match" and never activated the task.

Fix (two parts):
- SDK: coerce undefined to null in task input values before serialization,
  so they survive JSON.stringify and reach the Rust backend
- Rust: treat null in a partial as matching a missing key in the full
  config, so tasks correctly deactivate when the key is already absent

Assumption: null and undefined/absent are semantically equivalent for
StartOS config values. Input specs produce concrete values (strings,
numbers, booleans, objects, arrays) — null never appears as a meaningful
distinct-from-absent value in real-world configs.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

core/src/util/serde.rs

@@ -1456,7 +1456,8 @@ pub fn is_partial_of(partial: &Value, full: &Value) -> bool {
             if let Some(v_full) = full.get(k) {
                 is_partial_of(v, v_full)
             } else {
-                false
+                // null in partial matches a missing key in full (both represent absence)
+                v.is_null()
             }
         }),
         (Value::Array(partial), Value::Array(full)) => partial