[combobox] Moving focus to a chip closes the popup and discards the typed filter

## Current behavior

In a multi-select combobox with chips, moving focus onto a chip closes the popup and throws away the filter text that was typed.

Steps, using the `async-multiple` demo pattern (`Combobox.Chips` containing chips plus `Combobox.Input`, in `multiple` mode):

1. Click the input — the popup opens.
2. Pick an item so a chip appears next to the input.
3. Type a few characters to filter the list, e.g. `ba`. The popup stays open and shows the matches.
4. Press `ArrowLeft` until the caret reaches the start of the text.
5. Press `ArrowLeft` once more — focus moves to the last chip.

At step 5 the popup closes, and the `ba` typed in step 3 is discarded. Reopening the popup shows an empty input.

Reaching the same chip with the mouse does not appear to behave the same way: a pointer press on a chip is explicitly treated as interacting *within* the field, while arrowing to it is treated as leaving the field.

## Expected behavior

Moving focus onto a chip is a normal part of using the field, so it should not discard work in progress. If a user is partway through narrowing a long list and reaches for a chip to remove a wrong pick, the filter text should still be there afterwards.

Note that closing the popup on chip focus does appear to be deliberate — there is a test covering it (`closes the popup when a chip receives focus`), So this is not a request to keep the popup open unconditionally; the surprising parts are the lost filter text.

## Reproducible example

## Base UI version

v1.7.0 (`master`)

## Which browser are you using?

## Which OS are you using?

Windows

## Which assistive tech are you using (if applicable)?

N/A

## Additional context

Noticed while working on #5539.

Start with the Combobox.Chips and Combobox.Input handling used by the async-multiple demo, then run or read the existing test named “closes the popup when a chip receives focus.” Reproduce the ArrowLeft transition with a typed filter such as “ba”; done means the popup still closes as tested while the filter text remains available when the field is reopened.

Written by the indexing model from the issue text.