fix(ui5-shellbar): fix search-button-click event and getSearchButtonDomRef with ui5-shellbar-search

[plamenivanov91]

Both issues stem from the new ui5-shellbar-search path bypassing handleSearchButtonClick entirely ? the controller called setSearchState directly, which toggled the field but never fired the event.

search-button-click not fired (Issue 1)

• Add handleSearchButtonClick callback to ShellBarSearchConstructorParams and wire it up via getSearchDeps().
• Route onSearch in the ShellBarSearch controller through handleSearchButtonClick() instead of calling setSearchState directly, so the event fires consistently and preventDefault() is respected.
• Resolve the targetRef in handleSearchButtonClick from the ui5-shellbar-search shadow root (.ui5-shell-search-field-button) when isSelfCollapsibleSearch, so event.detail.targetRef is never null.

getSearchButtonDomRef returns null (Issue 2)

• Add getSearchButtonDomRef() to ShellBarSearch (the custom element), querying .ui5-shell-search-field-button from its own shadow root.
• Delegate from ShellBar.getSearchButtonDomRef() to the component method when isSelfCollapsibleSearch; legacy data-ui5-stable path is unchanged. Returns null when the search is expanded (button not in DOM) ? correct.

Tests (ShellBar.cy.tsx)

• fires search-button-click with non-null targetRef when ui5-shellbar-search is used.
• getSearchButtonDomRef returns the toggle button when collapsed.

Sample (ShellBar.html)

• Added a ui5-shellbar-search section with live event output and a button to call getSearchButtonDomRef() and display the result.

SNOW: DINC0975247
SNOW: DINC0975256

[sap-ui5-webcomponents-release]

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