fix: preserve multi-select ComboBox selections for custom input

[romansndlr]

Summary

• avoid clearing existing multi-select ComboBox values when allowsCustomValue commits or reverts custom input
• keep the custom input text while closing the menu for Escape, Enter, and Tab
• add a regression test covering the reported multi-select custom-value flow

Testing

• yarn jest packages/react-aria-components/test/ComboBox.test.js packages/react-stately/test/combobox/useComboBoxState.test.js --runInBand
• yarn eslint packages/react-stately/src/combobox/useComboBoxState.ts packages/react-aria-components/test/ComboBox.test.js

Closes #9837

[LFDanLu]

this seems to work well, will see about having the team test it fully in a upcoming test session. We may want to coordinate the "commitBehavior" prop addition with this PR since the Tab behavior does feel a bit odd with multiple selection IMO but that should probably be up to user discretion.