fix(ui5-multi-combobox): correct filtering when input doesn't match typeahead

[adrian-bobev]

Summary

When typing text that doesn't trigger typeahead autocomplete (e.g., "k" or "and"), the filter incorrectly used a stale valueBeforeAutoComplete instead of the actual typed value, causing wrong suggestions to appear.

Symptoms:

• Typing "k" showed all items instead of only "The United Kingdom..." (matching "Kingdom")
• Typing "and" showed items matching "a" (Albania, Argentina) instead of items with word "and"

Root Cause:
The condition this._shouldAutocomplete || !!autoCompletedChars caused filtering to use valueBeforeAutoComplete even when there was no actual autocomplete. The _shouldAutocomplete flag is true during normal typing, but valueBeforeAutoComplete only updates when typeahead finds a match.

Fix:
Only use valueBeforeAutoComplete when there are actually autocompleted characters selected (autoCompletedChars > 0).

Test Plan

• Added Cypress test for "k" scenario (no typeahead match, but filter match)
• Added Cypress test for "and" scenario (typeahead match on first char, then no match)
• All existing MultiComboBox tests pass

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[ui5-webcomponents-bot]

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