Keep a newly-typed value in a multiple select using a key_generator

[evnchn]

Motivation

ui.select({'a':'A'}, multiple=True, new_value_mode='add', key_generator=itertools.count(100)); user selects 'a', then types 'Banana' and presses Enter. QSelect emits [{'value':0,'label':'A'}, 'Banana']. The new option 100:'Banana' is added to options and appears in the dropdown, but the resulting value stays ['a'] -- the just-created chip vanishes from the selection instead of becoming ['a', 100].

Minimal reproduction (nicegui/elements/select.py:108) — the exact test/script that was run to reproduce it:

import itertools

from nicegui import ui
from nicegui.testing import User


def _model_value_listener(select):
    """The id of the element's 'update:model-value' listener (the one the client fires)."""
    return next(l.id for l in select._event_listeners.values() if l.type == 'update:modelValue')


async def test_multiple_select_drops_new_value_with_key_generator(user: User) -> None:
    """QSelect(multiple, new_value_mode='add', dict options + key_generator):
    selecting an existing option AND typing a NEW value adds the option but drops it from the value."""
    single = multiple = None

    @ui.page('/')
    def page():
        nonlocal single, multiple
        single = ui.select({'a': 'A'}, multiple=False, new_value_mode='add', key_generator=itertools.count(100))
        multiple = ui.select({'a': 'A'}, multiple=True, new_value_mode='add', key_generator=itertools.count(200))

    await user.open('/')

    # --- single-select control (WORKS): QSelect emits the raw typed string ---
    single._handle_event({'listener_id': _model_value_listener(single), 'args': 'Banana'})
    assert single.options == {'a': 'A', 100: 'Banana'}
    assert single.value == 100  # new key becomes the value -- correct

    # --- multiple-select (BROKEN): QSelect emits [existing-as-dict, new-as-str] ---
    multiple._handle_event({'listener_id': _model_value_listener(multiple),
                            'args': [{'value': 0, 'label': 'A'}, 'Banana']})
    assert multiple.options == {'a': 'A', 200: 'Banana'}  # option IS added (visible in dropdown)
    # BUG: value is ['a'] -- the just-created chip vanished. Should be ['a', 200].
    assert multiple.value == ['a', 200], f"new value dropped: got {multiple.value!r}, expected ['a', 200]"

→ F [100%]

Implementation

One-line fix in Select._event_args_to_value multiple branch: capture _handle_new_value's returned key into args[i] (mirrors single-select branch), so a newly-typed value with dict options + key_generator is kept in the selection.

Verification

• Fail-first A/B verified: FAILS on origin/main (['a'] != ['a',200]), PASSES with fix.
• Local gates: pre-commit · mypy ./nicegui · pylint 10.00/10 · pytest (touched) — all green.

Progress

• The PR title is a short phrase starting with a verb.
• The implementation is complete.
• This PR does not address a security issue.
• Pytests have been added/updated.
• Documentation is not necessary.
• No breaking changes to the public API.